D-Serine as a putative glial neurotransmitter

Chronic intermittent hypoxia enhances glycinergic inhibition in nucleus tractus solitarius

Chronic intermittent hypoxia (CIH), an animal model of sleep apnea, has been shown to alter the activity of second-order chemoreceptor neurons in the caudal nucleus of the solitary tract (cNTS). Although numerous studies have focused on excitatory plasticity, few studies have explored CIH-induced plasticity impacting inhibitory inputs to NTS neurons, and the roles of GABAergic and glycinergic inputs on heightened cNTS excitability following CIH are unknown. In addition, changes in astrocyte function may play a role in cNTS plasticity responses to CIH. This study tested the effects of a 7-day CIH protocol on miniature inhibitory postsynaptic currents (mIPSCs) in cNTS neurons receiving chemoreceptor afferents. Normoxia-treated rats primarily displayed GABA mIPSCs, whereas CIH-treated rats exhibited a shift toward combined GABA/glycine-mediated mIPSCs. CIH increased glycinergic mIPSC amplitude and area. This shift was not observed in dorsal motor nucleus of the vagus neurons or cNTS cells from females. Immunohistochemistry showed that strengthened glycinergic mIPSCs were associated with increased glycine receptor protein and were dependent on receptor trafficking in CIH-treated rats. In addition, CIH altered astrocyte morphology in the cNTS, and inactivation of astrocytes following CIH reduced glycine receptor-mediated mIPSC frequency and overall mIPSC amplitude. In cNTS, CIH produced changes in glycine signaling that appear to reflect increased trafficking of glycine receptors to the cell membrane. Increased glycine signaling in cNTS associated with CIH also appears to be dependent on astrocytes. Additional studies will be needed to determine how CIH influences glycine receptor expression and astrocyte function in cNTS.NEW & NOTEWORTHY Chronic intermittent hypoxia (CIH) has been used to mimic the hypoxemia associated with sleep apnea and determine how these hypoxemias influence neural function. The nucleus of the solitary tract is the main site for chemoreceptor input to the CNS, but how CIH influences NTS inhibition has not been determined. These studies show that CIH increases glycine-mediated miniature IPSCs through mechanisms that depend on protein trafficking and astrocyte activation.

Excitatory and inhibitory D-serine binding to the NMDA receptor

N-methyl-D-aspartate receptors (NMDARs) uniquely require binding of two different neurotransmitter agonists for synaptic transmission. D-serine and glycine bind to one subunit, GluN1, while glutamate binds to the other, GluN2. These agonists bind to the receptor's bi-lobed ligand-binding domains (LBDs), which close around the agonist during receptor activation. To better understand the unexplored mechanisms by which D-serine contributes to receptor activation, we performed multi-microsecond molecular dynamics simulations of the GluN1/GluN2A LBD dimer with free D-serine and glutamate agonists. Surprisingly, we observed D-serine binding to both GluN1 and GluN2A LBDs, suggesting that D-serine competes with glutamate for binding to GluN2A. This mechanism is confirmed by our electrophysiology experiments, which show that D-serine is indeed inhibitory at high concentrations. Although free energy calculations indicate that D-serine stabilizes the closed GluN2A LBD, its inhibitory behavior suggests that it either does not remain bound long enough or does not generate sufficient force for ion channel gating. We developed a workflow using pathway similarity analysis to identify groups of residues working together to promote binding. These conformation-dependent pathways were not significantly impacted by the presence of N-linked glycans, which act primarily by interacting with the LBD bottom lobe to stabilize the closed LBD.

Gliotransmission of D-serine promotes thirst-directed behaviors in Drosophila

Thirst emerges from a range of cellular changes that ultimately motivate an animal to consume water. Although thirst-responsive neuronal signals have been reported, the full complement of brain responses is unclear. Here, we identify molecular and cellular adaptations in the brain using single-cell sequencing of water-deprived Drosophila. Water deficiency primarily altered the glial transcriptome. Screening the regulated genes revealed astrocytic expression of the astray-encoded phosphoserine phosphatase to bi-directionally regulate water consumption. Astray synthesizes the gliotransmitter D-serine, and vesicular release from astrocytes is required for drinking. Moreover, dietary D-serine rescues aay-dependent drinking deficits while facilitating water consumption and expression of water-seeking memory. D-serine action requires binding to neuronal NMDA-type glutamate receptors. Fly astrocytes contribute processes to tripartite synapses, and the proportion of astrocytes that are themselves activated by glutamate increases with water deprivation. We propose that thirst elevates astrocytic D-serine release, which awakens quiescent glutamatergic circuits to enhance water procurement.

Neurotransmitters-Key Factors in Neurological and Neurodegenerative Disorders of the Central Nervous System

Neurotransmitters are molecules that amplify, transmit, and convert signals in cells, having an essential role in information transmission throughout the nervous system. Hundreds of such chemicals have been discovered in the last century, continuing to be identified and studied concerning their action on brain health. These substances have been observed to influence numerous functions, including emotions, thoughts, memories, learning, and movements. Thus, disturbances in neurotransmitters' homeostasis started being correlated with a plethora of neurological and neurodegenerative disorders. In this respect, the present paper aims to describe the most important neurotransmitters, broadly classified into canonical (e.g., amino acids, monoamines, acetylcholine, purines, soluble gases, neuropeptides) and noncanonical neurotransmitters (e.g., exosomes, steroids, D-aspartic acid), and explain their link with some of the most relevant neurological conditions. Moreover, a brief overview of the recently developed neurotransmitters' detection methods is offered, followed by several considerations on the modulation of these substances towards restoring homeostasis.

Chirality and the Origin of Life

The origin of life, based on the homochirality of biomolecules, is a persistent mystery. Did life begin by using both forms of chirality, and then one of the forms disappeared? Or did the choice of homochirality precede the formation of biomolecules that could ensure replication and information transfer? Is the natural choice of L-amino acids and D-sugars on which life is based deterministic or random? Is the handedness present in/of the Universe from its beginning? The whole biosystem on the Earth, all living creatures are chiral. Many theories try to explain the origin of life and chirality on the Earth: e.g., the panspermia hypothesis, the primordial soup hypothesis, theory of parity violation in weak interactions. Additionally, heavy neutrinos and the impact of the fact that only left-handed particles decay, and even dark matter, all have to be considered.

The Role of Amino Acids in Neurotransmission and Fluorescent Tools for Their Detection

Neurotransmission between neurons, which can occur over the span of a few milliseconds, relies on the controlled release of small molecule neurotransmitters, many of which are amino acids. Fluorescence imaging provides the necessary speed to follow these events and has emerged as a powerful technique for investigating neurotransmission. In this review, we highlight some of the roles of the 20 canonical amino acids, GABA and β-alanine in neurotransmission. We also discuss available fluorescence-based probes for amino acids that have been shown to be compatible for live cell imaging, namely those based on synthetic dyes, nanostructures (quantum dots and nanotubes), and genetically encoded components. We aim to provide tool developers with information that may guide future engineering efforts and tool users with information regarding existing indicators to facilitate studies of amino acid dynamics.

Microchemical identification of enantiomers in early-branching animals: Lineage-specific diversification in the usage of D-glutamate and D-aspartate

D-amino acids are unique and essential signaling molecules in neural, hormonal, and immune systems. However, the presence of D-amino acids and their recruitment in early animals is mostly unknown due to limited information about prebilaterian metazoans. Here, we performed the comparative survey of L-/D-aspartate and L-/D-glutamate in representatives of four phyla of early-branching Metazoa: cnidarians (Aglantha); placozoans (Trichoplax), sponges (Sycon) and ctenophores (Pleurobrachia, Mnemiopsis, Bolinopsis, and Beroe), which are descendants of ancestral animal lineages distinct from Bilateria. Specifically, we used high-performance capillary electrophoresis for microchemical assays and quantification of the enantiomers. L-glutamate and L-aspartate were abundant analytes in all species studied. However, we showed that the placozoans, cnidarians, and sponges had high micromolar concentrations of D-aspartate, whereas D-glutamate was not detectable in our assays. In contrast, we found that in ctenophores, D-glutamate was the dominant enantiomer with no or trace amounts of D-aspartate. This situation illuminates prominent lineage-specific diversifications in the recruitment of D-amino acids and suggests distinct signaling functions of these molecules early in the animal evolution. We also hypothesize that a deep ancestry of such recruitment events might provide some constraints underlying the evolution of neural and other signaling systems in Metazoa.

A Review of Neurotransmitters Sensing Methods for Neuro-Engineering Research

Neurotransmitters as electrochemical signaling molecules are essential for proper brain function and their dysfunction is involved in several mental disorders. Therefore, the accurate detection and monitoring of these substances are crucial in brain studies. Neurotransmitters are present in the nervous system at very low concentrations, and they mixed with many other biochemical molecules and minerals, thus making their selective detection and measurement difficult. Although numerous techniques to do so have been proposed in the literature, neurotransmitter monitoring in the brain is still a challenge and the subject of ongoing research. This article reviews the current advances and trends in neurotransmitters detection techniques, including in vivo sampling and imaging techniques, electrochemical and nano-object sensing techniques for in vitro and in vivo detection, as well as spectrometric, analytical and derivatization-based methods mainly used for in vitro research. The document analyzes the strengths and weaknesses of each method, with the aim to offer selection guidelines for neuro-engineering research.

The Energy Landscape of Human Serine Racemase

Human serine racemase is a pyridoxal 5′-phosphate (PLP)-dependent dimeric enzyme that catalyzes the reversible racemization of L-serine and D-serine and their dehydration to pyruvate and ammonia. As D-serine is the co-agonist of the N-methyl-D-aspartate receptors for glutamate, the most abundant excitatory neurotransmitter in the brain, the structure, dynamics, function, regulation and cellular localization of serine racemase have been investigated in detail. Serine racemase belongs to the fold-type II of the PLP-dependent enzyme family and structural models from several orthologs are available. The comparison of structures of serine racemase co-crystallized with or without ligands indicates the presence of at least one open and one closed conformation, suggesting that conformational flexibility plays a relevant role in enzyme regulation. ATP, Mg²⁺, Ca²⁺, anions, NADH and protein interactors, as well as the post-translational modifications nitrosylation and phosphorylation, finely tune the racemase and dehydratase activities and their relative reaction rates. Further information on serine racemase structure and dynamics resulted from the search for inhibitors with potential therapeutic applications. The cumulative knowledge on human serine racemase allowed obtaining insights into its conformational landscape and into the mechanisms of cross-talk between the effector binding sites and the active site.

PICK1 Genetic Variation and Cognitive Function in Patients with Schizophrenia

The gene of protein interacting with C kinase 1 alpha (PICK1) has been implicated in schizophrenia, nevertheless, conflicting results existed. However, its role in cognitive function remains unclear. Besides, cognitive deficits impair the long-term outcome. We explored whether the polymorphisms of PICK1 (rs2076369, rs3952) affected cognitive functions in schizophrenic patients. We analyzed 302 patients and tested the differences of cognitive functions, clinical symptoms between genetic groups. We also used general linear model to analyze the effect of PICK1 genetic polymorphisms on cognitive functions. After adjustment for gender, age, education, the patients with rs2076369 G/T genotype showed better performance than T/T homozygotes in the summary score, global composite score, neurocognitive composite score, category fluency subtest, WAIS-III-Digit Symbol Coding subtest, working memory, WMS-III-Spatial Span (backward) subtest, MSCEIT-managing emotions branch (p = 0.038, 0.025, 0.046, 0.036, 0.025, 0.027, 0.035, 0.028, respectively). G/G homozygotes performed better than T/T in category fluency subtest (p = 0.049). A/A homozygotes of rs3952 performed better than G/G in trail making A subtest (p = 0.048). To our knowledge, this is the first study to indicate that PICK1 polymorphisms may associate with cognitive functions in schizophrenic patients. Further replication studies in healthy controls or other ethnic groups are warranted.

D-Serine in the nucleus accumbens region modulates behavioral sensitization and extinction of conditioned place preference

BACKGROUND

D-serine, the endogenous co-agonist of N-methyl-D-aspartate receptors (NMDARs), is considered to be essential for learning and memory. The aim of the current investigation was to systematically evaluate the role of D-serine on addiction behaviors considered to be mediated by the nucleus accumbens (NAc).

METHODS

D-Serine concentration in the NAc was measured by high-performance liquid chromatography (HPLC). Cocaine-induced behavioral sensitization and conditioned place preference (CPP) models were used to evaluate the relation between changes in serine in the nucleus accumbens and cocaine-induced behavioral effects. The expression of serine racemase (SR), D-amino acid oxidase (DAAO), the cAMP response element-binding protein (CREB) and upstream kinases, and N-methyl-D-aspartate (NMDA) receptors subunits were analyzed by western blot. Long-term depression (LTD) in the NAc was investigated by electrophysiological methods.

RESULTS

The NAc slices obtained from the behavioral sensitization rats presented significantly reduced D-serine concentrations, increased expression of DAAO, and down-regulated expression of SR in a dose-dependent manner. Furthermore, D-serine injections into the nucleus accumbens blocked the development of behavioral sensitization and caused extinction of CPP. The ERK-CREB-Fos pathway and the NMDA receptor NR2B subunits in the NAc were involved in the cocaine-induced behavioral sensitization. We also found that D-serine was essential for NMDAR-dependent LTD and D-serine-regulated LTD in a bell-shaped concentration-dependent manner. The disrupted NMDAR-dependent LTD in the NAc of cocaine-treated rats was reversed by D-serine.

CONCLUSIONS

Our results provide evidence for a critical role of D-serine in synaptic plasticity relevant to cocaine addiction and indicate that D-serine may be an effective therapeutic agent for cocaine addiction.

Identifying the metabolic perturbations in earthworm induced by cypermethrin using gas chromatography-mass spectrometry based metabolomics

Globally, cypermethrin is one of the most widely used synthetic pyrethroid for agricultural and domestic purposes. Most part of the pesticides used in the agriculture ends up as residues in the soil, making soil dwelling organisms, especially earthworms more susceptible to pesticide intoxication. Cypermethrin is known to be a neurotoxicant to many model organisms, including mammals and insects, but such type of toxicity evidence is not available for invertebrate systems like earthworms. In the present work, metabolomics based approach was utilized to identify the toxic mechanism of action of cypermethrin on earthworm (Metaphire posthuma) and these were exposed to sub-lethal concentrations of cypermethrin such as 2.5 mg/kg, 5 mg/kg, 10 mg/kg and 20 mg/kg (1/40^th, 1/20^th, 1/10^th and 1/5^th of LC₅₀, respectively) for fourteen days. The results revealed that 22 metabolites (mainly fatty acids, sugars and amino acids) were shown significant responses in the exposed earthworms and these responses are dose dependent. It is proposed that mainly carbohydrate and fatty acids in neural system metabolism was disturbed. Overall, the results provided that metabolomics can be an effective tool to understand the effects of cypermethrin on the metabolic responses of earthworm Metaphire posthuma.

Neuronal Regulation of Neuroprotective Microglial Apolipoprotein E Secretion in Rat In Vitro Models of Brain Pathophysiology

Apolipoprotein E (ApoE) is mainly secreted by glial cells and is involved in many brain functions, including neuronal plasticity, β-amyloid clearance, and neuroprotection. Microglia--the main immune cells of the brain--are one source of ApoE, but little is known about the physiologic regulation of microglial ApoE secretion by neurons and whether this release changes under inflammatory or neurodegenerative conditions. Using rat primary neural cell cultures, we show that microglia release ApoE through a Golgi-mediated secretion pathway and that ApoE progressively accumulates in neuroprotective microglia-conditioned medium. This constitutive ApoE release is negatively affected by microglial activation both with lipopolysaccharide and with ATP. Microglial ApoE release is stimulated by neuron-conditioned media and under coculture conditions. Neuron-stimulated microglial ApoE release is mediated by serine and glutamate through N-methyl-D-aspartate receptors and is differently regulated by activation states (i.e. lipopolysaccharide vs ATP) and by 6-hydroxydopamine. Microglial ApoE silencing abrogated protection of cerebellar granule neurons against 6-hydroxydopamine toxicity in cocultures, indicating that microglial ApoE release is neuroprotective. Our findings shed light on the reciprocal cross-talk between neurons and microglia that is crucial for normal brain functions. They also open the way for the identification of possible pharmacologic targets that can modulate neuroprotective microglial ApoE release under pathologic conditions.

DISC1-TSNAX and DAOA genes in major depression and citalopram efficacy

BACKGROUND

Major depressive disorder (MDD) is a common disease with high morbidity and still unsatisfying treatment response. Both MDD pathogenesis and antidepressant effect are supposed to be strongly affected by genetic polymorphisms. Among promising candidate genes, distrupted in schizophrenia 1 (DISC1), translin-associated factor X (TSNAX) and D-amino acid oxidase activator (DAOA) were suggested since their regulator role in neurodevelopment, neuroplasticity and neurotransmission, and previous evidence of cross-involvement in major psychiatric diseases.

METHODS

The present paper investigated the role of 13 SNPs within the reported genes in MDD susceptibility through a case-control (n=320 and n=150, respectively) study and in citalopram efficacy (n=157). Measures of citalopram efficacy were response (4th week) and remission (12th week). Pharmacogenetic findings were tested in the STAR(⁎)D genome-wide dataset (n=1892) for replication.

RESULTS

Evidence of association among rs3738401 (DISC1), rs1615409 and rs766288 (TSNAX) and MDD was found (p=0.004, p=0.0019, and p=0.008, respectively). A trend of association between remission and DISC1 rs821616 and DAOA rs778294 was detected, and confirmation was found for rs778294 by repeated-measure ANOVA (p=0.0008). In the STAR(⁎)D a cluster of SNPs from 20 to 40Kbp from DISC1 findings in the original sample was associated with citalopram response, as well as rs778330 (12,325bp from rs778294).

LIMITATIONS

Relatively small size of the original sample and focus on only three candidate genes.

CONCLUSIONS

The present study supported a role of DISC1-TSNAX variants in MDD susceptibility. On the other hand, genetic regions around DAOA rs778294 and DISC1 rs6675281-rs1000731 may influence citalopram efficacy.

TRPA1 channels are regulators of astrocyte basal calcium levels and long-term potentiation via constitutive D-serine release

Astrocytes are found throughout the brain where they make extensive contacts with neurons and synapses. Astrocytes are known to display intracellular Ca(2+) signals and release signaling molecules such as D-serine into the extracellular space. However, the role(s) of astrocyte Ca(2+) signals in hippocampal long-term potentiation (LTP), a form of synaptic plasticity involved in learning and memory, remains unclear. Here, we explored a recently discovered novel TRPA1 channel-mediated transmembrane Ca(2+) flux pathway in astrocytes. Specifically, we determined whether block or genetic deletion of TRPA1 channels affected LTP of Schaffer collateral to CA1 pyramidal neuron synapses. Using pharmacology, TRPA1(-/-) mice, imaging, electrophysiology, and D-serine biosensors, our data indicate that astrocyte TRPA1 channels contribute to basal Ca(2+) levels and are required for constitutive D-serine release into the extracellular space, which contributes to NMDA receptor-dependent LTP. The findings have broad relevance for the study of astrocyte-neuron interactions by demonstrating how TRPA1 channel-mediated fluxes contribute to astrocyte basal Ca(2+) levels and neuronal function via constitutive D-serine release.

Fractalkine (CX3CL1) enhances hippocampal N-methyl-D-aspartate receptor (NMDAR) function via D-serine and adenosine receptor type A2 (A2AR) activity

BACKGROUND

N-Methyl-D-aspartate receptors (NMDARs) play fundamental roles in basic brain functions such as excitatory neurotransmission and learning and memory processes. Their function is largely regulated by factors released by glial cells, including the coagonist d-serine. We investigated whether the activation of microglial CX3CR1 induces the release of factors that modulate NMDAR functions.

METHODS

We recorded the NMDAR component of the field excitatory postsynaptic potentials (NMDA-fEPSPs) elicited in the CA1 stratum radiatum of mouse hippocampal slices by Shaffer collateral stimulation and evaluated D-serine content in the extracellular medium of glial primary cultures by mass spectrometry analysis.

RESULTS

We demonstrated that CX3CL1 increases NMDA-fEPSPs by a mechanism involving the activity of the adenosine receptor type A2 (A2AR) and the release of the NMDAR coagonist D-serine. Specifically (1) the selective A2AR blocker 7-(2-phenylethyl)-5-amino-2-(2-furyl)-pyrazolo-[4,3-e]-1,2,4-triazolo[1,5-c]pyrimidine (SCH58261) and the genetic ablation of A2AR prevent CX3CL1 action while the A2AR agonist 5-(6-amino-2-(phenethylthio)-9H-purin-9-yl)-N-ethyl-3,4-dihydroxytetrahydrofuran-2-carboxamide (VT7) mimics CX3CL1 effect, and (2) the selective blocking of the NMDAR glycine (and D-serine) site by 5,7-dicholorokynurenic acid (DCKA), the enzymatic degradation of D-serine by D-amino acid oxidase (DAAO) and the saturation of the coagonist site by D-serine, all block the CX3CL1 effect. In addition, mass spectrometry analysis demonstrates that stimulation of microglia and astrocytes with CX3CL1 or VT7 increases D-serine release in the extracellular medium.

CONCLUSIONS

CX3CL1 transiently potentiates NMDAR function though mechanisms involving A2AR activity and the release of D-serine.

In vivo D-serine hetero-exchange through alanine-serine-cysteine (ASC) transporters detected by microelectrode biosensors

D-serine, a co-agonist of N-methyl D-aspartate (NMDA) receptors, has been implicated in neurological and psychiatric disorders such as cerebral ischemia, lateral amyotrophic sclerosis, or schizophrenia. D-serine signaling represents an important pharmacological target for treating these diseases; however, the biochemical mechanisms controlling extracellular D-serine levels in vivo are still unclear. D-serine heteroexchange through small neutral amino acid transporters has been shown in cell cultures and brain slices and could provide a biochemical mechanism for the control of D-serine extracellular concentration in vivo. Alternatively, exocytotic D-serine release has also been proposed. In this study, the dynamics of D-serine release and clearance were explored in vivo on a second-by-second time scale using microelectrode biosensors. The rate of D-serine clearance in the rat frontal cortex after a microionophoretic injection revealed a transporter-mediated uptake mechanism. D-serine uptake was blocked by small neutral l-amino acids, implicating alanine-serine-cysteine (ASC) transporters, in particular high affinity Asc-1 and low affinity ASCT2 transporters. Interestingly, changes in alanine, serine, or threonine levels resulted in D-serine release through ASC transporters. Asc-1, but not ASCT2, appeared to release D-serine in response to changes in amino acid concentrations. Finally, neuronal silencing by tetrodotoxin increased D-serine extracellular concentration by an ASC-transporter-dependent mechanism. Together, these results indicate that D-serine heteroexchange through ASC transporters is present in vivo and may constitute a key component in the regulation of D-serine extracellular concentration.

Analysis of neurotransmitter amino acids by CE-LIF detection in biological fluids

Neurotransmitter amino acids like Alanine (Ala), Glutamic acid (Glu), Aspartic acid (Asp), Serine (Ser), Taurine (Tau), and Glycine (Gly) are widely distributed in biological fluids and tissues and they have important physiological functions, so that the evaluation of their levels in organisms plays an essential role in physiology. We here report a new capillary electrophoresis (CE) method with laser-induced fluorescence detection by which analytes are resolved and detected in less than 12 min both in human plasma and in other samples types, such as red blood cells, urine, cultured cells, cerebrospinal fluid, saliva, and vitreous humor, thus avoiding the typical limitations of the other methods normally suitable only in one or two matrix types.

Characterization of human DAAO variants potentially related to an increased risk of schizophrenia

Considering the key role of d-serine in N-methyl-d-aspartate receptor-mediated neurotransmission, it is highly relevant to define the role that enzymes play in d-serine synthesis and degradation. In particular, the details of regulation of the d-serine catabolic human enzyme d-amino acid oxidase (hDAAO) are unknown although different lines of evidence have shown it to be involved in schizophrenia susceptibility. Here we investigated the effect of three single nucleotide polymorphisms and known mutations in hDAAO, i.e., D31H, R279A, and G331V. A very low amount of soluble G331V hDAAO is produced in E. coli cells: the recombinant variant enzyme is fully active. Human U87 glioblastoma cells transiently transfected for G331V hDAAO show a low viability, a significant amount of protein aggregates, and augmented apoptosis. The recombinant D31H and R279A hDAAO variants do not show alterations in tertiary and quaternary structures, thermal stability, binding affinity for inhibitors, and the modulator pLG72, whereas the kinetic efficiency and the affinity for d-serine and for FAD were higher than for the wild-type enzyme. While these effects for the substitution at position 31 cannot be structurally explained, the R279A mutation might affect the hDAAO FAD-binding affinity by altering the "structurally ambivalent" peptide V47-L51. In agreement with the observed increased activity, expression of D31H and R279A hDAAO variants in U87 cells produces a higher decrease in cellular d/(d+l) serine ratio than the wild-type counterpart. In vivo, these substitutions could affect cellular d-serine concentration and its release at synapsis and thus might be relevant for schizophrenia susceptibility.

Genotypic association of the DAOA gene with resting-state brain activity in major depression

Compelling evidence suggests that the glutamatergic system may contribute to the pathophysiology of major depression (MDD). While the D-amino acid oxidase activator (DAOA) gene can affect glutamatergic function, its genetic associations with MDD and abnormal resting-state brain activity have yet to be elucidated. A total of 488 patients with MDD and 480 controls were recruited to examine MDD association for the DAOA gene in a Chinese population, of whom 53 medication-free patients and 46 well-matched controls underwent resting-state functional magnetic resonance imaging for regional homogeneity (ReHo) analysis. The differences in ReHo between genotypes of interest were initially tested by the Student's t test, and the 2 × 2 (genotypes × disease status) ANOVA was then performed to identify the main effects of genotypes, disease status, and their interactions in MDD. Allelic association of the DAOA gene with MDD was observed for rs2391191, rs3918341, and rs778294 and haplotypic association for 2- and 3-SNP haplotypes. Six clusters in the cerebellum, right middle frontal gyrus and left middle temporal gyrus showed genotypic association between altered ReHo and rs2391191. The main effects of rs2391191 genotypes were found in the right culmen and right middle frontal gyrus. The left uvula and left middle temporal gyrus showed a genotypes × disease status interaction. Our results suggest that the DAOA gene may confer genetic risk of MDD. Genotypic effect of rs2391191 and its interaction with disease status may contribute to the altered ReHo in patients with MDD. Glutamatergic modulation may play an important role in alteration of the resting-state brain activities.

D-serine adjuvant treatment alleviates behavioural and motor symptoms in Parkinson's disease

Parkinson's disease (PD) manifestations include motor symptoms and behavioural deficits that resemble schizophrenia negative symptoms. The N-methyl-D-aspartate subtype of glutamate receptor (NMDAR) represents a novel pharmacological target in PD. D-serine (DSR) allosterically modulates in-vivo NMDAR-mediated neurotransmission and has been shown to improve negative and antipsychotic drug-induced parkinsonian symptoms in schizophrenia patients. This pilot study assessed DSR effects in ten PD patients who completed a 6-wk double-blind, placebo-controlled, crossover adjuvant treatment trial with 30 mg/kg.d DSR. Primary outcome analyses consisted of separate repeated-measures multivariate analyses of variance for Unified Parkinson's Disease Rating Scale (UPDRS), Simpson-Angus Scale for Extrapyramidal Symptoms (SAS), Abnormal Involuntary Movement Scale (AIMS), and Positive and Negative Syndrome Scale (PANSS) scores. DSR treatment was well tolerated and resulted in increased DSR serum levels (p=0.001) and significantly reduced UPDRS (p=0.02), SAS (p=0.009) and PANSS (0.05) total scores. These preliminary findings suggest that DSR treatment may be beneficial in PD. Larger-sized studies with optimized DSR dosages are warranted.

EphrinBs regulate D-serine synthesis and release in astrocytes

There is growing evidence that astrocytes play critical roles in neuron-glial interactions at the synapse. Astrocytes are believed to regulate presynaptic and postsynaptic structures and functions, in part, by the release of gliotransmitters such as glutamate, ATP, and d-serine; however, little is known of how neurons and astrocytes communicate to regulate these processes. Here, we investigated a family of transmembrane proteins called ephrinBs and Eph receptors that are expressed in the synapse and are known to regulate synaptic transmission and plasticity. In addition to their presence on CA1 hippocampal neurons, we determined that ephrins and Eph receptors are also expressed on hippocampal astrocytes. Stimulation of hippocampal astrocytes with soluble ephrinB3, known to be expressed on CA1 postsynaptic dendrites, enhanced d-serine synthesis and release in culture. Conversely, ephrinB3 had no effect on d-serine release from astrocytes deficient in EphB3 and EphA4, which are the primary receptors for ephrinB3. Eph receptors mediate this response through interactions with PICK1 (protein interacting with C-kinase) and by dephosphorylating protein kinase C α to activate the conversion of l-serine to d-serine by serine racemase. These findings are supported in vivo, where reduced d-serine levels and synaptic transmissions are observed in the absence of EphB3 and EphA4. These data support a role for ephrins and Eph receptors in regulating astrocyte gliotransmitters, which may have important implications on synaptic transmission and plasticity.

Glutamate receptor ion channels: structure, regulation, and function

The mammalian ionotropic glutamate receptor family encodes 18 gene products that coassemble to form ligand-gated ion channels containing an agonist recognition site, a transmembrane ion permeation pathway, and gating elements that couple agonist-induced conformational changes to the opening or closing of the permeation pore. Glutamate receptors mediate fast excitatory synaptic transmission in the central nervous system and are localized on neuronal and non-neuronal cells. These receptors regulate a broad spectrum of processes in the brain, spinal cord, retina, and peripheral nervous system. Glutamate receptors are postulated to play important roles in numerous neurological diseases and have attracted intense scrutiny. The description of glutamate receptor structure, including its transmembrane elements, reveals a complex assembly of multiple semiautonomous extracellular domains linked to a pore-forming element with striking resemblance to an inverted potassium channel. In this review we discuss International Union of Basic and Clinical Pharmacology glutamate receptor nomenclature, structure, assembly, accessory subunits, interacting proteins, gene expression and translation, post-translational modifications, agonist and antagonist pharmacology, allosteric modulation, mechanisms of gating and permeation, roles in normal physiological function, as well as the potential therapeutic use of pharmacological agents acting at glutamate receptors.

Quantification of neurotransmitter amino acids by capillary electrophoresis laser-induced fluorescence detection in biological fluids

The role of neurotransmitter amino acids (NAAs) in the functioning of the nervous system has been the focus of increasingly intense research over the past several years. Among the various amino acids that have important roles as neurotransmitters, there are alanine (Ala), glutamic acid (Glu), aspartic acid (Asp), serine (Ser), taurine (Tau) and glycine (Gly). NAAs are present in plasma, cells and--at trace levels--in all biological fluids, but complex components in biological matrices make it difficult to determine them in biological samples. We describe a new capillary electrophoresis (CE) method with laser-induced fluorescence detection by which analytes are resolved in less than 12 minutes in a 18 mmol/L phosphate run buffer at pH 11.6. The use of elevated temperatures during sample derivatization leads to a drastic reduction in the reaction time, down to 20 min, compared to the 6-14 h usually described for reactions between FITC and amino acids at room temperature. In order to demonstrate its wide range of applications, the method was applied to the analysis of NAA in human plasma and in other sample types, such as red blood cells, urine, cultured cells, cerebrospinal fluid, saliva and vitreous humor, thus avoiding the typical limitations of other methods, which are normally suitable for use with only one or two matrix types.

Astrocytes and human cognition: modeling information integration and modulation of neuronal activity

Recent research focusing on the participation of astrocytes in glutamatergic tripartite synapses has revealed mechanisms that support cognitive functions common to human and other mammalian species, such as learning, perception, conscious integration, memory formation/retrieval and the control of voluntary behavior. Astrocytes can modulate neuronal activity by means of release of glutamate, d-serine, adenosine triphosphate and other signaling molecules, contributing to sustain, reinforce or depress pre- and post-synaptic membranes. We review molecular mechanisms present in tripartite synapses and model the cognitive role of astrocytes. Single protoplasmic astrocytes operate as a "Local Hub", integrating information patterns from neuronal and glial populations. Two mechanisms, here modeled as the "domino" and "carousel" effects, contribute to the formation of intercellular calcium waves. As waves propagate through gap junctions and reach other types of astrocytes (interlaminar, polarized, fibrous and varicose projection), the active astroglial network functions as a "Master Hub" that integrates results of distributed processing from several brain areas and supports conscious states. Response of this network would define the effect exerted on neuronal plasticity (membrane potentiation or depression), behavior and psychosomatic processes. Theoretical results of our modeling can contribute to the development of new experimental research programs to test cognitive functions of astrocytes.

D-Serine metabolism in C6 glioma cells: Involvement of alanine-serine-cysteine transporter (ASCT2) and serine racemase (SRR) but not D-amino acid oxidase (DAO)

D-serine is an endogenous N-methyl-D-aspartate (NMDA) receptor coagonist. It is synthesized from L-serine by serine racemase (SRR), but many aspects of its metabolism remain unclear, especially in the forebrain, which lacks active D-amino acid oxidase (DAO), the major D-serine degradative enzyme. Candidate mechanisms include SRR operating in α,β-eliminase mode (converting D-serine to pyruvate) and regulation by serine transport, in which the alanine-serine-cysteine transporter ASCT2 is implicated. Here we report studies in C6 glioma cells, which “simulate” the forebrain, in that the cells express SRR and ASCT2 but lack DAO activity. We measured D-serine, ASCT2, SRR, and DAO expression and DAO activity in two situations: after incubation of cells for 48 hr with serine isomers and after increased or decreased SRR expression by transfection and RNA interference, respectively. Incubation with serine enantiomers decreased [³H]D-serine uptake and ASCT2 mRNA and increased SRR immunoreactivity but did not alter DAO immunoreactivity, and DAO activity remained undetectable. SRR overexpression increased D-serine and pyruvate and decreased [³H]D-serine uptake and ASCT2 mRNA but did not affect DAO. SRR knockdown did not alter any of the parameters. Our data suggest that D-serine transport mediated by ASCT2 contributes prominently to D-serine homeostasis when DAO activity is absent. The factors regulating D-serine are important for understanding normal NMDA receptor function and because D-serine, along with DAO and SRR, is implicated in the pathogenesis and treatment of schizophrenia. © 2010 Wiley-Liss, Inc.

Identification of novel glial genes by single-cell transcriptional profiling of Bergmann glial cells from mouse cerebellum

Bergmann glial cells play critical roles in the structure and function of the cerebellum. During development, their radial processes serve as guides for migrating granule neurons and their terminal endfeet tile to form the glia limitans. As the cerebellum matures, Bergmann glia perform important roles in synaptic transmission and synapse maintenance, while continuing to serve as essential structural elements. Despite growing evidence of the diverse functions of Bergmann glia, the molecular mechanisms that mediate these functions have remained largely unknown. As a step toward identifying the molecular repertoire underlying Bergmann glial function, here we examine global gene expression in individual Bergmann glia from developing (P6) and mature (P30) mouse cerebellum. When we select for developmentally regulated genes, we find that transcription factors and ribosomal genes are particularly enriched at P6 relative to P30; whereas synapse associated molecules are enriched at P30 relative to P6. We also analyze genes expressed at high levels at both ages. In all these categories, we find genes that were not previously known to be expressed in glial cells, and discuss novel functions some of these genes may potentially play in Bergmann glia. We also show that Bergmann glia, even in the adult, express a large set of genes thought to be specific to stem cells, suggesting that Bergmann glia may retain neural precursor potential as has been proposed. Finally, we highlight several genes that in the cerebellum are expressed in Bergmann glia but not astrocytes, and may therefore serve as new, specific markers for Bergmann glia.

Pilot controlled trial of D-serine for the treatment of post-traumatic stress disorder

Enhancement of neurotransmission mediated at N-methyl-D-aspartate subtype of glutamate receptors (NMDAR) may be beneficial in post-traumatic stress disorder (PTSD). d-serine (DSR) is an endogenous full agonist at the NMDAR-associated glycine modulatory site. Twenty-two chronic PTSD outpatients were randomly assigned to participate in a 6-wk double-blind, placebo-controlled, crossover trial with 30 mg/kg x d DSR used as monotherapy or add-on pharmacotherapy. Outcome was assessed using the Clinician-Administered PTSD scale (CAPS), Hamilton Anxiety (HAMA) and Depression (HAMD) scales and the civilian version of the Mississippi Scale for Combat-Related PTSD (MISS). DSR treatment was well tolerated and resulted in significantly (p=0.03) increased DSR serum levels. Compared with placebo administration, DSR treatment resulted in significantly reduced HAMA (p=0.007) and MISS (p=0.001) scores and a trend (p=0.07) towards improved CAPS total scores. These preliminary findings indicate that NMDAR glycine site-based pharmacotherapy may be effective in PTSD and warrant larger-sized clinical trials with optimized DSR dosages.

Glial-neuronal interactions--implications for plasticity and drug addiction

Among neuroscientists, astrocytes have for long played Cinderella to their neuron stepsisters. While the importance of glia in regulating brain activity was predicted by Ramon y Cajal more than a century ago (Garcia-Marin et al., Trends. Neurosci. 30:479-787, 2007), these cells, until recently, have been thought to play mainly a passive part in synaptic signaling. Results obtained over the last decade have begun to suggest otherwise. Experiments carried out in a number of labs have shown that glial cells, especially astrocytes, directly participate in synaptic signaling and potentially regulate synaptic plasticity and network excitability. The presence of signaling pathways on astrocytes that are analogous to those at presynaptic terminals suggests a role for these cells in network plasticity. Findings that the same signaling pathways can be activated by receptors for drugs of abuse present on astrocytes suggest a role for these cells in the addictive process. In this review, we summarize current understanding of astrocytic role in synaptic signaling and suggest that a complete understanding of the process of addiction requires a better understanding of the functional role of these cells.

Glutamatergic regulation of serine racemase via reversal of PIP2 inhibition

D-serine is a physiologic coagonist with glutamate at NMDA-subtype glutamate receptors. As D-serine is localized in glia, synaptically released glutamate presumably stimulates the glia to form and release D-serine, enabling glutamate/D-serine cotransmission. We show that serine racemase (SR), which generates D-serine from L-serine, is physiologically inhibited by phosphatidylinositol (4,5)-bisphosphate (PIP2) presence in membranes where SR is localized. Activation of metabotropic glutamate receptors (mGluR5) on glia leads to phospholipase C-mediated degradation of PIP2, relieving SR inhibition. Thus mutants of SR that cannot bind PIP2 lose their membrane localizations and display a 4-fold enhancement of catalytic activity. Moreover, mGluR5 activation of SR activity is abolished by inhibiting phospholipase C.

The mystery and magic of glia: a perspective on their roles in health and disease

In this perspective, I review recent evidence that glial cells are critical participants in every major aspect of brain development, function, and disease. Far more active than once thought, glial cells powerfully control synapse formation, function, and blood flow. They secrete many substances whose roles are not understood, and they are central players in CNS injury and disease. I argue that until the roles of nonneuronal cells are more fully understood and considered, neurobiology as a whole will progress only slowly.

The astrocyte odyssey

Neurons have long held the spotlight as the central players of the nervous system, but we must remember that we have equal numbers of astrocytes and neurons in the brain. Are these cells only filling up the space and passively nurturing the neurons, or do they also contribute to information transfer and processing? After several years of intense research since the pioneer discovery of astrocytic calcium waves and glutamate release onto neurons in vitro, the neuronal-glial studies have answered many questions thanks to technological advances. However, the definitive in vivo role of astrocytes remains to be addressed. In addition, it is becoming clear that diverse populations of astrocytes coexist with different molecular identities and specialized functions adjusted to their microenvironment, but do they all belong to the umbrella family of astrocytes? One population of astrocytes takes on a new function by displaying both support cell and stem cell characteristics in the neurogenic niches. Here, we define characteristics that classify a cell as an astrocyte under physiological conditions. We will also discuss the well-established and emerging functions of astrocytes with an emphasis on their roles on neuronal activity and as neural stem cells in adult neurogenic zones.

D-amino acids in the brain: the biochemistry of brain serine racemase

It has been recently established that in various brain regions D-serine, the product of serine racemase, occupies the so-called 'glycine site' within N-methyl D-aspartate receptors. Mammalian brain serine racemase is a pyridoxal-5' phosphate-containing enzyme that catalyzes the racemization of L-serine to D-serine. It has also been shown to catalyze the alpha,beta-elimination of water from L-serine or D-serine to form pyruvate and ammonia. Serine racemase is included within the group of type II-fold pyridoxal-5' phosphate enzymes, together with many other racemases and dehydratases. Serine racemase was first purified from rat brain homogenates and later recombinantly expressed in mammalian and insect cells as well as in Escherichia coli. It has been shown that serine racemase is activated by divalent cations like calcium, magnesium and manganese, as well as by nucleotides like ATP, ADP or GTP. In turn, serine racemase is also strongly inhibited by reagents that react with free sulfhydryl groups such as glutathione. Several yeast two-hybrid screens for interaction partners identified the proteins glutamate receptor interacting protein, protein interacting with C kinase 1 and Golga3 to bind to serine racemase, having different effects on its catalytic activity or stability. In addition, it has also been proposed that serine racemase is regulated by phosphorylation. Thus, d-serine production in the brain is tightly regulated by various factors pointing at its physiologic importance. In this minireview, we will focus on the regulation of brain serine racemase and d-serine synthesis by the factors mentioned above.

Astrocyte and neuron intone through glutamate

The unexpected finding of astrocytes to release glutamate as gliotransmitter challenges the traditional concepts on astrocyte being "passive" in CNS communications. Glutamate is the major excitatory transmitter in transferring information between neurons, but is now also known to activate astrocyte through transporters and receptors. Together with the sensitive swelling response, astrocytes could respond directly to glutamate and neuronal activity. Other new functions of astrocytes include modulation of synaptic plasticity and cerebral blood flow (CBF). The classic glutamate deplenishment through glutamine synthesis and CO(2) production does not account for the total glutamate internalized into astrocytes. This leads us to speculate there are many hidden functions of glutamate in neurons and astrocytes waiting to be discovered. In this review, we attempted to reexamine some of these new and older functions of glutamate and to reevaluate the roles of glutamate intoning these two cell types.

Cloning and functional characterization of Arabidopsis thaliana D-amino acid aminotransferase--D-aspartate behavior during germination

The understanding of D-amino acid metabolism in higher plants lags far behind that in mammals, for which the biological functions of these unique amino acids have already been elucidated. In this article, we report on the biochemical behavior of D-amino acids (particularly D-Asp) and relevant metabolic enzymes in Arabidopsis thaliana. During germination and growth of the plant, a transient increase in D-Asp levels was observed, suggesting that D-Asp is synthesized in the plant. Administration of D-Asp suppressed growth, although the inhibitory mechanism responsible for this remains to be clarified. Exogenous D-Asp was efficiently incorporated and metabolized, and was converted to other D-amino acids (D-Glu and D-Ala). We then studied the related metabolic enzymes, and consequently cloned and characterized A. thaliana D-amino acid aminotransferase, which is presumably involved in the metabolism of D-Asp in the plant by catalyzing transamination between D-amino acids. This is the first report of cDNA cloning and functional characterization of a D-amino acid aminotransferase in eukaryotes. The results presented here provide important information for understanding the significance of D-amino acids in the metabolism of higher plants.

D-serine signalling as a prominent determinant of neuronal-glial dialogue in the healthy and diseased brain

Rather different from their initial image as passive supportive cells of the CNS, the astrocytes are now considered as active partners at synapses, able to release a set of gliotransmitter-like substances to modulate synaptic communication within neuronal networks. Whereas glutamate and ATP were first regarded as main determinants of gliotransmission, growing evidence indicates now that the amino acid D-serine is another important player in the neuronal-glial dialogue. Through the regulation of glutamatergic neurotransmission through both N-methyl-D-aspartate (NMDA-R) and non-NMDA-R, D-serine is helping in modelling the appropriate connections in the developing brain and influencing the functional plasticity within neuronal networks throughout lifespan. The understanding of D-serine signalling, which has increased linearly in the last few years, gives new insights into the critical role of impaired neuronal-glial communication in the diseased brain, and offers new opportunities for developing relevant strategies to treat cognitive deficits associated to brain disorders.

Regulation of N-methyl-D-aspartate receptors by astrocytic D-serine

NMDA receptors (NMDARs) are key glutamatergic receptors in the CNS. Their permeability to Ca2+ and their voltage-dependent Mg2+ block make them essential for synaptic transmission, synaptic plasticity, rhythmogenesis, gene expression and excitotoxicity. One very peculiar property is that their activation requires the binding of both glutamate and a co-agonist like glycine or D-serine. There is a growing body of evidence indicating that D-serine, rather than glycine as originally thought, is the endogenous ligand for NMDARs in many brain structures. D-serine is synthesized mainly in glial cells and it is released upon activation of glutamate receptors. Its concentration in the synaptic cleft controls the number of NMDAR available for activation by glutamate. Consequently, the glial environment of neurons has a critical impact on the direction and magnitude of NMDAR-dependent synaptic plasticity.

Roles of l-serine and sphingolipid synthesis in brain development and neuronal survival

Sphingolipids represent a class of membrane lipids that contain a hydrophobic ceramide chain as its common backbone structure. Sphingolipid synthesis requires two simple components: l-serine and palmitoyl CoA. Although l-serine is classified as a non-essential amino acid, an external supply of l-serine is essential for the synthesis of sphingolipids and phosphatidylserine (PS) in particular types of central nervous system (CNS) neurons. l-Serine is also essential for these neurons to undergo neuritogenesis and to survive. Biochemical analysis has shown that l-serine is synthesized from glucose and released by astrocytes but not by neurons, which is the major reason why this amino acid is an essential amino acid for neurons. Biosynthesis of membrane lipids, such as sphingolipids, PS, and phosphatidylethanolamine (PE), in neurons is completely dependent on this astrocytic factor. Recent advances in lipid biology research using transgenic mice have demonstrated that synthesis of endogenous l-serine and neuronal sphingolipids is essential for brain development. In this review, we discuss the metabolic system that coordinates sphingolipid synthesis with the l-serine synthetic pathway between neurons and glia. We also discuss the crucial roles of the metabolic conversion of l-serine to sphingolipids in neuronal development and survival. Human diseases associated with serine and sphingolipid biosynthesis are also discussed.

The serine racemase mRNA is predominantly expressed in rat brain neurons

D-serine is an endogenous and obligatory coagonist for the glycine site of the N-methyl-D-aspartate receptor in the mammalian brain. D-serine is synthesized from L-serine by serine racemase; immunohistochemical studies have long been believed to indicate that serine racemase and D-serine occur predominantly in astrocytes. However, we have recently demonstrated in the primary cultures that both the mRNA and protein levels of serine racemase are higher in neurons than in astrocytes. Here we report the application of in situ hybridization based on tyramide signal amplification for the detection of serine racemase mRNA in sections of the adult rat brain. Serine racemase mRNA could be demonstrated in a large number of neurons throughout the brain, especially in the forebrain such as the cerebral cortex, striatum, and hippocampus. This is the first study to demonstrate the exact localization of serine racemase mRNA at the cellular or tissue level. These results suggest that neuron-derived D-serine could modulate neurotransmission via the glycine site of N-methyl-D-aspartate receptors.

Insights into the activation of brain serine racemase by the multi-PDZ domain glutamate receptor interacting protein, divalent cations and ATP

Brain serine racemase contains pyridoxal phosphate as a prosthetic group and is known to become activated by divalent cations such as Ca(2+) and Mg(2+), as well as by ATP and ADP. In vivo, brain serine racemase is also activated by a multi-PSD-95/discs large/ZO-1 (PDZ) domain glutamate receptor interacting protein (GRIP) that is usually coupled to the GluR2/3 subunits of the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid Ca(2+) channel. In the present study, we analysed the mechanisms by which serine racemase becomes activated by GRIP, divalent cations and ATP. We show that binding of PDZ6 of GRIP to serine racemase does not result in increased d-serine production. However, full-length GRIP does augment significantly enzymatic activity. We expressed various GRIP shorter constructs to map down the regions within GRIP that are necessary for serine racemase activation. We observed that, whereas recombinant proteins containing PDZ4-PDZ5-PDZ6 are unable to activate serine racemase, other constructs containing PDZ4-PDZ5-PDZ6-GAP2-PDZ7 significantly augment its activity. Hence, activation of serine racemase by GRIP is not a direct consequence of the translocation towards the calcium channel but rather a likely conformational change induced by GRIP on serine racemase. On the other hand, the observed activation of serine racemase by divalent cations has been assumed to be a side-effect associated with ATP binding, which is known to form a complex with Mg(2+) ions. Because no mammalian serine racemase has yet been crystallized, we used molecular modelling based on yeast and bacterial homologs to demonstrate that the binding sites for Ca(2+), ATP and the PDZ6 domain of GRIP are spatially separated and modulate the enzyme through distinct mechanisms.