Glial uptake of intracerebroventricularly injected D-serine in the rat brain: an immunocytochemical study

Racemization in Post-Translational Modifications Relevance to Protein Aging, Aggregation and Neurodegeneration: Tip of the Iceberg

Homochirality of DNA and prevalent chirality of free and protein-bound amino acids in a living organism represents the challenge for modern biochemistry and neuroscience. The idea of an association between age-related disease, neurodegeneration, and racemization originated from the studies of fossils and cataract disease. Under the pressure of new results, this concept has a broader significance linking protein folding, aggregation, and disfunction to an organism's cognitive and behavioral functions. The integrity of cognitive function is provided by a delicate balance between the evolutionarily imposed molecular homo-chirality and the epigenetic/developmental impact of spontaneous and enzymatic racemization. The chirality of amino acids is the crucial player in the modulation the structure and function of proteins, lipids, and DNA. The collapse of homochirality by racemization is the result of the conformational phase transition. The racemization of protein-bound amino acids (spontaneous and enzymatic) occurs through thermal activation over the energy barrier or by the tunnel transfer effect under the energy barrier. The phase transition is achieved through the intermediate state, where the chirality of alpha carbon vanished. From a thermodynamic consideration, the system in the homo-chiral (single enantiomeric) state is characterized by a decreased level of entropy. The oscillating protein chirality is suggesting its distinct significance in the neurotransmission and flow of perceptual information, adaptive associative learning, and cognitive laterality. The common pathological hallmarks of neurodegenerative disorders include protein misfolding, aging, and the deposition of protease-resistant protein aggregates. Each of the landmarks is influenced by racemization. The brain region, cell type, and age-dependent racemization critically influence the functions of many intracellular, membrane-bound, and extracellular proteins including amyloid precursor protein (APP), TAU, PrP, Huntingtin, α-synuclein, myelin basic protein (MBP), and collagen. The amyloid cascade hypothesis in Alzheimer's disease (AD) coexists with the failure of amyloid beta (Aβ) targeting drug therapy. According to our view, racemization should be considered as a critical factor of protein conformation with the potential for inducing order, disorder, misfolding, aggregation, toxicity, and malfunctions.

D-serine reduces memory impairment and neuronal damage induced by chronic lead exposure

Although exogenous D-serine has been applied as a neural regulatory intervention in many studies, the role played by D-serine in hippocampal injuries caused by lead exposure remains poorly understood. Rat models of chronic lead exposure were established through the administration of 0.05% lead acetate for 8 weeks. Simultaneously, rats were administered 30 or 60 mg/kg D-serine, intraperitoneally, twice a day. Our results showed that D-serine treatment shortened the escape latency from the Morris water maze, increased the number of times that mice crossed the original platform location, and alleviated the pathological damage experienced by hippocampal neurons in response to lead exposure. Although D-serine administration did not increase the expression levels of the N-methyl-D-aspartate receptor subtype 2B (NR2B) in the hippocampi of lead-exposed rats, 60 mg/kg D-serine treatment restored the expression levels of NR2A, which are reduced by lead exposure. These findings suggested that D-serine can alleviate learning and memory impairments induced by lead exposure and that the underlying mechanism is associated with the increased expression of NR2A in the hippocampus. This study was approved by the Animal Ethics Committee of North China University of Science and Technology, China (approval No. LX2018155) on December 21, 2018.

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.

Astrocyte calcium signaling transforms cholinergic modulation to cortical plasticity in vivo

Global brain state dynamics regulate plasticity in local cortical circuits, but the underlying cellular and molecular mechanisms are unclear. Here, we demonstrate that astrocyte Ca(2+) signaling provides a critical bridge between cholinergic activation, associated with attention and vigilance states, and somatosensory plasticity in mouse barrel cortex in vivo. We investigated first whether a combined stimulation of mouse whiskers and the nucleus basalis of Meynert (NBM), the principal source of cholinergic innervation to the cortex, leads to enhanced whisker-evoked local field potential. This plasticity is dependent on muscarinic acetylcholine receptors (mAChR) and N-methyl-d-aspartic acid receptors (NMDARs). During the induction of this synaptic plasticity, we find that astrocytic [Ca(2+)](i) is pronouncedly elevated, which is blocked by mAChR antagonists. The elevation of astrocytic [Ca(2+)](i) is crucial in this type of synaptic plasticity, as the plasticity could not be induced in inositol-1,4,5-trisphosphate receptor type 2 knock-out (IP(3)R2-KO) mice, in which astrocytic [Ca(2+)](i) surges are diminished. Moreover, NBM stimulation led to a significant increase in the extracellular concentration of the NMDAR coagonist d-serine in wild-type mice when compared to IP(3)R2-KO mice. Finally, plasticity in IP(3)R2-KO mice could be rescued by externally supplying d-serine. Our data present coherent lines of in vivo evidence for astrocytic involvement in cortical plasticity. These findings suggest an unexpected role of astrocytes as a gate for cholinergic plasticity in the cortex.

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 Regulation of NMDA Receptor Activity

The N-Methyl-D-aspartate-type glutamate receptor (NMDAR) plays a key role in several important processes involving the nervous system, including brain development, synaptic plasticity, and learning. Unlike other neurotransmitter receptors, which are activated by individual neurotransmitters, activation of NMDARs requires the binding of a coagonist (D-serine or glycine) in addition to glutamate. Although previously considered an "unnatural" amino acid, D-serine is a key regulator of NMDAR activity and may be the main physiological ligand at the coagonist site. D-Serine is synthesized in the mammalian brain and is enriched in astrocytes, a class of glial cells that ensheath synapses in the brain. Astrocytes physiologically affect NMDAR neurotransmission by releasing D-serine, suggesting that D-serine acts as a gliotransmitter. However, recent findings indicate that D-serine signaling does not depend solely on glia, because D-serine and its biosynthetic enzyme are also present in substantial amounts in neurons. Here, we discuss these new findings, which begin to shed light on the relative roles of glia and neurons in D-serine signaling.

Evidence for involvement of glial cell activity in the control of extracellular D-serine contents in the rat brain

The continuous intra-cortical infusion of a glia toxin, fluorocitrate, at the concentration of 1 mM caused a decrease in the cortical extracellular contents of an intrinsic coagonist for the N-methyl-D-aspartate (NMDA) type glutamate receptor, D-serine, by peaking at 40 min by -25% but produced an increase in those of glycine and L-serine. The attenuated glial activity by fluorocitrate was verified by a marked reduction in the extracellular glutamine contents. The present findings suggest that a group of glial cells such as a population of the protoplasmic astrocytes could, at least in part, participate differently in the regulation of the extracellular release of D-serine and another NMDA coagonist glycine in the medial frontal cortex of the rat.

Immunohistochemical evidence for the localization of neurons containing the putative transmitter L-proline in rat brain

We examined whether there are the neurotransmitter candidate amino acid L-proline containing neurons localized in the rat brain. Antibodies against L-proline conjugated with rabbit serum albumin were raised in a rabbit and purified with affinity chromatography. Strong L-proline-like immunoreactivity was confined to several groups of neurons in the arcuate nucleus (n) and supraoptic n in the hypothalamus and area postrema. The brainstem had markedly stained fibers in the medial longitudinal fasciculus and localized neuronal cell body labeling in the red n, mesencephalic trigeminal n, lateral reticular n, raphe obscurus n, solitary n, compact ambiguus n, motor trigeminal n and n of trapezoid body. Our findings are consistent with the hypothesis that L-proline may function as a neurotransmitter or neuromodulator in the brain.

Molecular determinants ofD-serine-mediated gliotransmission: From release to function

Since the late 80s, it is recognized that functional activation of N-methyl D-aspartate receptors (NMDARs) requires the binding of both glutamate and glycine. However, the surprising discovery that the wrong isomer of serine, D-serine, is present in mammals has profoundly challenged this dogmatic model of NMDARs activation. Indeed, there are accumulating evidence indicating that D-serine is the endogenous ligand for the glycine modulatory binding site in many brain areas. D-Serine is synthesized in glial cells by serine racemase (SR) and released upon activation of glutamate receptors. Here, we will provide an overview of recent findings on the molecular and cellular mechanisms involved in the synthesis and release of this gliotransmitter. We will also emphasize the function of this novel messenger in regulating synaptic excitatory transmission and plasticity in different brain areas. Because it fulfils all criteria for a gliotransmitter, D-serine regulatory action on glutamatergic transmission further illustrates the emerging concept of the "tripartite synapse".

Cloning of a D-serine-regulated transcript dsr-2 from rat cerebral neocortex

D-serine is now considered to be an endogenous co-agonist of the NMDA receptor in mammalian brain. To obtain insight into the molecular mechanisms underlying D-serine metabolism and function, we explored transcripts that are responsive to D-serine in the neocortex of the 8-day-old infant rat by a differential cloning technique, RNA arbitrarily primed PCR. We isolated a novel D-serine inducible transcript, D-serine-responsive transcript-2 (dsr-2), that was exclusively expressed in the brain. Sequence analysis of the corresponding cDNAs to the transcript revealed that the dsr-2 mRNA consists of 7199 nucleotides with an open reading frame encoding 111 amino acids. The dsr-2 gene was located on the reverse strand within an intron of the neurexin-3alpha gene, mapped to rat chromosome 6q24-31. The regional distribution of the basal expression of dsr-2 and its ontogenic changes in the brain closely correlated with those of free D-serine and of NMDA receptor R2B subunit mRNA, but were somewhat different from those of the neurexin-3alpha transcript. These findings suggest that dsr-2 may be involved in D-serine metabolism and/or function, and in the interactions between D-serine, NMDA receptor and neurexin-3alpha, in mammalian brain.

Inhibition of D-serine accumulation in the Xenopus oocyte by expression of the rat ortholog of human 3'-phosphoadenosine 5'-phosphosulfate transporter gene isolated from the neocortex as D-serine modulator-1

D-serine in mammalian brains has been suggested to be an endogenous co-agonist of the NMDA-type glutamate receptor. We have explored the molecules regulating D-serine uptake and release from the rat neocortex cDNA library using a Xenopus oocyte expression system, and isolated a cDNA clone designated as dsm-1 (D-serine modulator-1) encoding a protein that reduces the accumulation of D-serine to the oocyte. dsm-1 is the rat orthologue of the human 3'-phosphoadenosine 5'-phosphosulfate transporter 1 (PAPST1) gene. The hydropathy analysis of the deduced amino acid sequence of the Dsm-1 protein predicts the 10 transmembrane domains with a long hydrophobic stretch in the C-terminal like some amino acid transporters. The dsm-1 mRNA is predominantly expressed in the forebrain areas that are enriched with D-serine and NMDA receptors, and in the liver. The transient expression of dsm-1 in COS-7 cells demonstrates a partially Golgi apparatus-related punctuate distribution throughout the cytoplasm with a concentration near the nucleus. dsm-1-expressing oocytes diminishes the sodium-dependent and -independent accumulation of D-serine and the basal levels of the intrinsic D-serine and increases the rate of release of the pre-loaded D-serine. These findings indicate that dsm-1 may, at least in part, be involved in the D-serine translocation across the vesicular or plasma membranes in the brain, and thereby control the extra- and intracellular contents of D-serine.

d-Serine uptake by isolated retinas is consistent with ASCT-mediated transport

Uptake of the neuromodulator D-serine by isolated larval tiger salamander (Ambystoma tigrinum) retinas was measured using capillary electrophoresis (CE). Excised retinas were incubated in Ringer's solution in the presence of 5 microM D-serine. The supernatant was removed after 30 min, mixed with 4-fluoro-7-nitrobenz-2-oxa-1,3-diazole (NBD-F) to fluorescently label amines and analyzed using CE. Significant D-serine uptake was observed over a period of 1.5h. This is the first observation of D-serine uptake by an intact retinal tissue. D-Serine uptake in the retina was Na(+)-dependent and blocked by l-alanine, l-threonine, and l-cysteine. This pharmacology is consistent with the sodium dependent heteroexchange expected of system ASC-type transporters.

D-amino acids in the central nervous system in health and disease

Recent evidence has shown that d-amino acids are present in animals and humans in high concentrations and fulfill specific biological functions. In the central nervous system, two d-amino acids, d-serine and d-aspartate, occur in considerable concentrations. d-Serine is synthesized and metabolized endogenously and the same might account for d-aspartate. d-Serine has been studied most extensively and was shown to play a role in excitatory amino acid metabolism, being a co-agonist of the N-methyl-d-aspartate (NMDA) receptor. Insight into d-serine metabolism is relevant for physiological NMDA receptor (NMDAr) activation and for all the disorders associated with an altered function of the NMDAr, such as schizophrenia, ischemia, epilepsy, and neurodegenerative disorders. d-Aspartate appears to play a role in development and endocrine function, but the precise function of d-aspartate and other d-amino acids in animals and humans requires further investigation. As d-amino acids play biological roles, alterations in the concentrations of d-amino acids might occur in some disorders and relate to the pathogenesis of these disorders. d-Amino acid concentrations may then not only help in the diagnostic process, but also provide novel therapeutic targets. Consequently, the presence and important roles of d-amino acids in higher organisms do not only challenge former theories on mammalian physiology, but also contribute to exciting new insights in human disease.

Serine racemase modulates intracellular D-serine levels through an alpha,beta-elimination activity

Mammalian brain contains high levels of d-serine, an endogenous co-agonist of N-methyl D-aspartate type of glutamate receptors. D-Serine is synthesized by serine racemase, a brain enriched enzyme converting L- to D-serine. Degradation of D-serine is achieved by D-amino acid oxidase, but this enzyme is not present in forebrain areas that are highly enriched in D-serine. We now report that serine racemase catalyzes the degradation of cellular D-serine itself, through the alpha,beta-elimination of water. The enzyme also catalyzes water alpha,beta-elimination with L-serine and L-threonine. alpha,beta-Elimination with these substrates is observed both in vitro and in vivo. To investigate further the role of alpha,beta-elimination in regulating cellular D-serine, we generated a serine racemase mutant displaying selective impairment of alpha,beta-elimination activity (Q155D). Levels of D-serine synthesized by the Q155D mutant are several-fold higher than the wild-type both in vitro and in vivo. This suggests that the alpha,beta-elimination reaction limits the achievable D-serine concentration in vivo. Additional mutants in vicinal residues (H152S, P153S, and N154F) similarly altered the partition between the alpha,beta-elimination and racemization reactions. alpha,beta-Elimination also competes with the reverse serine racemase reaction in vivo. Although the formation of L- from D-serine is readily detected in Q155D mutant-expressing cells incubated with physiological D-serine concentrations, reversal with wild-type serine racemase-expressing cells required much higher D-serine concentration. We propose that alpha,beta-elimination provides a novel mechanism for regulating intracellular D-serine levels, especially in brain areas that do not possess D-amino acid oxidase activity. Extracellular D-serine is more stable toward alpha,beta-elimination, likely due to physical separation from serine racemase and its elimination activity.

The N-methyl D-aspartate receptor glycine site and D-serine metabolism: an evolutionary perspective

The N-methyl D-aspartate (NMDA) type of glutamate receptor requires two distinct agonists to operate. Glycine is assumed to be the endogenous ligand for the NMDA receptor glycine site, but this notion has been challenged by the discovery of high levels of endogenous d-serine in the mammalian forebrain. I have outlined an evolutionary framework for the appearance of a glycine site in animals and the metabolic events leading to high levels of D-serine in brain. Sequence alignments of the glycine-binding regions, along with the scant experimental data available, suggest that the properties of invertebrate NMDA receptor glycine sites are probably different from those in vertebrates. The synthesis of D-serine in brain is due to a pyridoxal-5'-phosphate (B(6))-requiring serine racemase in glia. Although it remains unknown when serine racemase first evolved, data concerning the evolution of B(6) enzymes, along with the known occurrences of serine racemases in animals, point to D-serine synthesis arising around the divergence time of arthropods. D-Serine catabolism occurs via the ancient peroxisomal enzyme d-amino acid oxidase (DAO), whose ontogenetic expression in the hindbrain of mammals is delayed until the postnatal period and absent from the forebrain. The phylogeny of D-serine metabolism has relevance to our understanding of brain ontogeny, schizophrenia and neurotransmitter dynamics.

Distribution and pharmacology of alanine-serine-cysteine transporter 1 (asc-1) in rodent brain

A polyclonal antibody against the Na+-independent alanine-serine-cysteine transporter 1 (asc-1) was raised and the specificity of the antibody verified by Western blots performed on membranes prepared from HEK293 cells transiently transfected with the cloned murine asc-1. The antibody was then used to localize the transporter in the brain of two rodent species by using immunohistochemistry at the light and electron microscopical level. asc-1-immunoreactivity (asc-1-ir) was widely distributed throughout the mouse and rat brain. Areas with high levels of asc-1-ir included hypothalamus, the medial septal area, globus pallidus, entopeduncular nucleus, cingulate and retrosplenial cortices. Moderate asc-1-ir was observed in several areas including layers III and V of the neocortex, thalamus, nucleus accumbens, caudate putamen, bed nucleus of stria terminalis, all amygdaloid nuclei, hippocampus (CA1-CA3 and hilus of the dentate gyrus), as well as several brainstem nuclei. asc-1-ir was observed as punctuate staining consistent with varicosities matching neuronal cell bodies and dendritic fields. At the ultrastructural level, asc-1-ir was mainly confined to presynaptic terminals. Immunostaining in either glial cell bodies or perivascular sites was not observed and white matter was completely devoid of asc-1-ir. Furthermore, the pharmacology of the Na+-independent uptake site for [3H]d-serine in rat brain synaptosomal P2 fractions was compared with the substrate specificity of the cloned human asc-1 transporter and a high degree of correlation was demonstrated. We conclude that asc-1-ir is widespread in the brain and limited to neuronal structures and that asc-1 may contribute to synaptic clearance of d-serine in brain.

8-nitroguanine formation in the liver of hamsters infected with Opisthorchis viverrini

Nucleic acid damage by reactive nitrogen and oxygen species may contribute to the carcinogenesis associated with chronic infection and inflammation. We examined 8-nitroguanine and 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG) formation and nitric oxide (NO) production in hamsters infected with Opisthorchis viverrini (OV). Formation of 8-nitroguanine was assessed immunohistochemically with an antibody specific for 8-nitroguanine. 8-nitroguanine formation was found mainly in the cytoplasm and slightly in the nucleus of inflammatory cells and epithelial lining of bile duct at inflammatory areas in the liver. 8-nitroguanine immunoreactivity reached the highest intensity on day 30. A time profile of 8-nitroguanine formation was closely associated with that of plasma nitrate/nitrite. HPLC with an electrochemical detector revealed that the amount of 8-oxodG in the liver reached the maximal level on day 21. The mechanisms of 8-oxodG and 8-nitroguanine formation via O2*- and NO production triggered by OV infection were discussed in relation to cholangiocarcinoma development.

Effect of the intracerebroventricular and systemic administration of L-serine on the concentrations of D- and L-serine in several brain areas and periphery of rat

To gain further insight into the metabolic mechanism of endogenous D-serine, the effect of the intracerebroventricular and intraperitoneal administration of L-serine on the concentrations of D- and L-serine in several brain areas and periphery was investigated. The intracerebroventricular injection of L-serine caused a rapid and marked increase in the L-serine levels in almost all brain regions of adult rats. This administration also produced a gradual increase in the D-serine levels in the forebrain, whereas a slight but significant elevation of D-serine was found in the cerebellum and pons-medulla. The intraperitoneal administration of L-serine caused a marked increase in the L-serine levels in all brain regions of both infant and adult rats. The treatment induced a significant augmentation of the D-serine levels in all brain regions of infant rats with higher concentrations in the cerebellum and cortex, whereas no significant change was observed in the cerebellum and pons-medulla of adult rats. These in vivo observation, together with the fact that immunohistochemical studies have indicated that both D-serine and serine racemase are highly concentrated in Bergmann glia of developing cerebellum, suggest that D-serine can be synthesized not only in the forebrain but also in the hindbrain by serine racemase. Furthermore, because the drastic decline in the cerebellar D-serine level coincides well with a dramatic increase in the cerebellar D-amino acid oxidase during early postnatal development, synthesized D-serine may be metabolized by D-amino acid oxidase in the hindbrain of adult rats.

Glial transport of the neuromodulator D-serine

D-Serine is an endogenous agonist of NMDA receptors that occurs in astrocytes in gray matter areas of the brain. D-Serine is synthesized from L-serine by the activity of a glial enriched serine racemase, but little is known on the properties of D-serine transport and factors regulating its synaptic concentration. In the present report we characterize the transport of D-serine in astrocytes. In primary astrocyte cultures, D-serine uptake is dependent on sodium ions and exhibits both low affinity and low specificity for D-serine. The kinetics of D-serine transport resembles that of ASCT type transporters as several small neutral amino acids strongly inhibit the uptake of D-serine. D-Serine fluxes are coupled to counter-movement of L-serine and to a less extent to other small neutral amino acids. Thus, addition of D-serine to cell cultures elicits robust efflux of intracellular L-serine. Conversely, physiological concentrations of L-serine induce efflux of preloaded D-serine from astrocytes. L-Serine was more effective than kainate, which have been previously shown to induce D-serine release from astrocytes upon stimulation of non-NMDA type of glutamate receptors. The features of D-serine transport we describe reveal possible new mechanisms controlling the synaptic concentration of D-serine.

Uptake of D-serine by synaptosomal P2 fraction isolated from rat brain

We have demonstrated that the crude synaptosome P2 fraction prepared from the rat brain is able to accumulate [(3)H]D-serine in a saturable, temperature-dependent and partially sodium- and potassium-dependent manner with an affinity of a hundred micromolar range. The inhibition profile of D-serine accumulation by various amino acids is different from those of uptake systems reported for glycine and other amino acids. The present data suggest that the endogenous D-serine may be taken up mainly through a carrier-mediated transport system to regulate its extracellular concentration in the mammalian brain.

Immunohistochemical evidences for localization and production of D-serine in some neurons in the rat brain

D-Amino acids are thought not to occur in mammalian tissues. However, previous studies reported D-serine was present only in astrocytes in the rat brain. In the present study, it was indicated by a highly sensitive immunocytochemical method with a D-serine specific antibody that D-serine was contained not only in astrocytes but also in some neurons, such as pyramidal neurons in the cerebral cortex, and neurons in the nucleus of the trapezoid body. Some amacrine cells also showed strong immunoreactivity for D-serine in the eyes which were injected with colchicine into the corpus vitreum.

Effects of N-methyl-D-aspartate, kainate or veratridine on extracellular concentrations of free D-serine and L-glutamate in rat striatum: an in vivo microdialysis study

Using an in vivo microdialysis technique, we have investigated the effect of N-methyl-D-aspartate (NMDA) or kainate on the extracellular concentrations of free D-serine and L-glutamate in the striatum. A intrastriatal perfusion of NMDA or kainate caused a significant increase in the extracellular release of L-glutamate, but a significant decrease in that of D-serine. Co-perfusion of an NMDA receptor antagonist, MK-801, or an alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid/kainate receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), with NMDA or kainate significantly reversed the NMDA- or kainate-induced decrease in the extracellular level of D-serine, respectively. The NMDA- or kainate-evoked increase in the extracellular L-glutamate level was also reversed by co-perfusion of MK-801 or CNQX, respectively. Because D-serine acts as a potent and selective agonist for the glycine site of the NMDA receptor and because intracerebroventricularly injected D-serine is accumulated in the astrocytes, D-serine could be taken up by the astrocytes following synaptic activation. Furthermore, because cortical ablation to remove corticostriatal glutamatergic inputs attenuates the excitotoxic effects of kainate in the striatum, L-glutamate may enhance its own release through a presynaptic NMDA and/or non-NMDA receptor-mediated mechanism.

Uptake of D- and L-serine in C6 glioma cells

Cultured C6 glioma cells were able to accumulate [3H]D- and [3H]L-serine in a temperature- and Na+-dependent and saturable manner. The kinetic analysis of these accumulation phenomena indicates that the D- or L-serine uptake into the glioma cells might occur by a single-component system with an apparent Km value around 2480 microM (for D-serine) or 110 microM (for L-serine) and a Vmax value around 7 nmol/min per mg protein. The ratio of the L- to D-serine uptake affinity was similar to that previously reported in the astrocytes of rat cerebrum. The inhibition profiles of D- and L-serine uptake by various amino acid and related compounds resembled each other and those of the ASCT-like neutral amino acid transporter. The present findings therefore suggest that C6 glioma cells may accumulate D- and L-serine through a common ASCT-like transporter and provide a useful model for the glial uptake of extracellular D- and L-serine in the brain.

Distribution of D-amino acid oxidase (DAO) activity in the medulla and thoracic spinal cord of the rat: implications for a role for D-serine in autonomic function

The activity and regional distribution of D-amino acid oxidase (DAO), an enzyme that inactivates D-serine, were examined in the medulla and spinal cord of the rat by biochemical and histochemical procedures. DAO activity was noticeably low or absent in the nucleus of the solitary tract, ventrolateral medulla and intramediolateral cell column of the spinal cord. This may be indicative of a neuromodulatory role for endogenous D-serine (at the NMDA-glycine site) in in the central control of blood pressure.

Free D-aspartate and D-serine in the mammalian brain and periphery

It has long been assumed that L-forms of amino acids exclusively constitute free amino acid pools in mammals. However, a variety of studies in the last decade has demonstrated that free D-aspartate and D-serine occur in mammals and may have important physiological function in mammals. Free D-serine is confined predominantly to the forebrain structure, and the distribution and development of D-serine correspond well with those of the N-methyl-D-aspartate (NMDA)-type excitatory amino acid receptor. As D-serine acts as a potent and selective agonist for the strychnine-insensitive glycine site of the NMDA receptor, it is proposed that D-serine is a potential candidate for an NMDA receptor-related glycine site agonist in mammalian brain. In contrast, widespread and transient emergence of a high concentration of free D-aspartate is observed in the brain and periphery. Since the periods of maximal emergence of D-aspartate in the brain and periphery occur during critical periods of morphological and functional maturation of the organs, D-aspartate could participate in the regulation of these regulation of these developmental processes of the organs. This review deals with the recent advances in the studies of presence of free D-aspartate and D-serine and their metabolic systems in mammals. Since D-aspartate and D-serine have been shown to potentiate NMDA receptor-mediated transmission through the glutamate binding site and the strychnine-insensitive glycine binding site, respectively, and have been utilized extensively as potent and selective tools to study the excitatory amino acid system in the brain, we shall discuss also the NMDA receptor and uptake system of D-amino acids.

Analytical chemistry and biochemistry of D-amino acids

The methodologies for the analysis of D-amino acids in biological materials have been reviewed, including the use of enzymes, gas and liquid chromatography with chiral stationary phases and diastereomer derivatization with chiral reagents followed by GC or HPLC separation. The distribution of D-amino acids in the body, their origin, metabolism and possible roles in human diseases are discussed.

Direct immunocytochemical evidence for the transfer of glutamine from glial cells to neurons: use of specific antibodies directed against the d-stereoisomers of glutamate and glutamine

We have raised antibodies against D-stereoisomers of the amino acids glutamate and glutamine. These stereoisomers are not naturally occurring in mammals but can be taken up into cells by transporters that normally handle the endogenous L-amino acids. Exposure of isolated rabbit retinae to 50 microM D-glutamate resulted in a strong accumulation of D-glutamate, and hence immunoreactivity for D-glutamate in radial glial cells (Müller cells). By contrast the glutamatergic ganglion cells exhibited no immunoreactivity for D-glutamate. D-Glutamate can be converted into D-glutamine by the glial enzyme glutamine synthetase. Immunolabelling for D-glutamine revealed the presence of D-glutamine in somata of subsets of neurons including the glutamatergic ganglion cells. Labelling was also present in the inner plexiform layer, possibly indicating labelling of neuronal processes. These data indicate that after D-glutamate has been taken up into glial cells it is converted into D-glutamine. This D-glutamine is then exported from the glial cells and taken up by a subset of neurons, including the glutamatergic ganglion cells.