Sensitive high-performance liquid chromatographic assay for D-amino-acid oxidase activity in mammalian tissues using a fluorescent non-natural substrate, 5-fluoro-D-tryptophan

Chemogenetic emulation of intraneuronal oxidative stress affects synaptic plasticity

Oxidative stress, a state of disrupted redox signaling, reactive oxygen species (ROS) overproduction, and oxidative cell damage, accompanies numerous brain pathologies, including aging-related dementia and Alzheimer's disease, the most common neurodegenerative disorder of the elderly population. However, a causative role of neuronal oxidative stress in the development of aging-related cognitive decline and neurodegeneration remains elusive because of the lack of approaches for modeling isolated oxidative injury in the brain. Here, we present a chemogenetic approach based on the yeast flavoprotein d-amino acid oxidase (DAAO) for the generation of intraneuronal hydrogen peroxide (H₂O₂). To validate this chemogenetic tool, DAAO and HyPer7, an ultrasensitive genetically encoded H₂O₂ biosensor, were targeted to neurons. Changes in the fluorescence of HyPer7 upon treatment of neurons expressing DAAO with d-norvaline (D-Nva), a DAAO substrate, confirmed chemogenetically induced production of intraneuornal H₂O₂. Then, using the verified chemogenetic tool, we emulated isolated intraneuronal oxidative stress in acute brain slices and, using electrophysiological recordings, revealed that it does not alter basal synaptic transmission and the probability of neurotransmitter release from presynaptic terminals but reduces long-term potentiation (LTP). Moreover, treating neurons expressing DAAO with D-Nva via the patch pipette also decreases LTP. This observation indicates that isolated oxidative stress affects synaptic plasticity at single cell level. Our results broaden the toolset for studying normal redox regulation in the brain and elucidating the role of oxidative stress to the pathogenesis of cognitive aging and the early stages of aging-related neurodegenerative diseases. The proposed approach is useful for identification of early markers of neuronal oxidative stress and may be used in screens of potential antioxidants effective against neuronal oxidative injury.

Mouse d-Amino-Acid Oxidase: Distribution and Physiological Substrates

d-Amino-acid oxidase (DAO) catalyzes the oxidative deamination of d-amino acids. DAO is present in a wide variety of organisms and has important roles. Here, we review the distribution and physiological substrates of mouse DAO. Mouse DAO is present in the kidney, brain, and spinal cord, like DAOs in other mammals. However, in contrast to other animals, it is not present in the mouse liver. Recently, DAO has been detected in the neutrophils, retina, and small intestine in mice. To determine the physiological substrates of mouse DAO, mutant mice lacking DAO activity are helpful. As DAO has wide substrate specificity and degrades various d-amino acids, many d-amino acids accumulate in the tissues and body fluids of the mutant mice. These amino acids are d-methionine, d-alanine, d-serine, d-leucine, d-proline, d-phenylalanine, d-tyrosine, and d-citrulline. Even in wild-type mice, administration of DAO inhibitors elevates D-serine levels in the plasma and brain. Among the above d-amino acids, the main physiological substrates of mouse DAO are d-alanine and d-serine. These two d-amino acids are most abundant in the tissues and body fluids of mice. d-Alanine derives from bacteria and produces bactericidal reactive oxygen species by the action of DAO. d-Serine is synthesized by serine racemase and is present especially in the central nervous system, where it serves as a neuromodulator. DAO is responsible for the metabolism of d-serine. Since DAO has been implicated in the etiology of neuropsychiatric diseases, mouse DAO has been used as a representative model. Recent reports, however, suggest that mouse DAO is different from human DAO with respect to important properties.

D-Serine Production, Degradation, and Transport in ALS: Critical Role of Methodology

In mammalian systems, D-serine is perhaps the most biologically active D-amino acid described to date. D-serine is a coagonist at the NMDA-receptor, and receptor activation is dependent on D-serine binding. Because D-serine binding dramatically increases receptor affinity for glutamate, it can produce excitotoxicity without any change in glutamate per se. D-serine is twofold higher in the spinal cords of mSOD1 (G93A) ALS mice, and the deletion of serine racemase (SR), the enzyme that produces D-serine, results in an earlier onset of symptoms, but with a much slower rate of disease progression. Localization studies within the brain suggest that mSOD1 and subsequent glial activation could contribute to the alterations in SR and D-serine seen in ALS. By also degrading both D-serine and L-serine, SR appears to be a prime bidirectional regulator of free serine levels in vivo. Therefore, accurate and reproducible measurements of D-serine are critical to understanding its regulation by SR. Several methods for measuring D-serine have been employed, and significant issues related to validation and standardization remain unresolved. Further insights into the intracellular transport and tissue-specific compartmentalization of D-serine within the CNS will aid in the understanding of the role of D-serine in the pathogenesis of ALS.

Kynurenic acid and 3-hydroxykynurenine production from D-kynurenine in mice

Kynurenic acid (KYNA), an antagonist of the α7 nicotinic acetylcholine receptor and the N-methyl-D-aspartate receptor, and 3-hydroxykynurenine (3-HK), a generator of reactive oxygen species, are neuroactive metabolites of the kynurenine pathway of tryptophan degradation. In the mammalian brain as elsewhere, both compounds derive from a common bioprecursor, L-kynurenine (L-KYN). Recent studies in rats demonstrated that D-kynurenine (D-KYN), a metabolite of the bacterial amino acid D-tryptophan, can also function as a bioprecursor of brain KYNA. We now investigated the conversion of systemically administered D-KYN to KYNA in mice and also explored the possible production of 3-HK in the same animals. Thirty min after an injection of D-KYN or L-KYN (30 mg/kg, i.p.), newly produced KYNA and 3-HK were recovered from plasma, liver, forebrain and cerebellum in all cases. Using a new chiral separation method, 3-HK produced from D-KYN was positively identified as D-3-HK. L-KYN was the more effective precursor of KYNA in all tissues and also exceeded D-KYN as a precursor of brain 3-HK. In contrast, D-KYN was more potent as a precursor of 3-HK in the liver. The production of both KYNA and 3-HK from D-KYN was rapid in all tissues, peaking at 15-30 min following a systemic injection of D-KYN. These results show that biosynthetic routes other than those classically ascribed to L-KYN can account for the synthesis of both KYNA and 3-HK in vivo. This new insight may be of significant physiological or pathological relevance.

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

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

Determination of D-serine and D-alanine in the tissues and physiological fluids of mice with various D-amino-acid oxidase activities using two-dimensional high-performance liquid chromatography with fluorescence detection

Two-dimensional-HPLC procedures have been established for the sensitive and selective determination of D-serine (D-Ser) and D-alanine (D-Ala), and their amounts in the tissues and physiological fluids of mice with various D-amino-acid oxidase (DAO) activities have been demonstrated. These two D-amino acids are modulators of the N-methyl-D-aspartate receptor mediated neurotransmission, and the alterations in their amounts following the changes in the DAO activity are matters of interest. After pre-column derivatization with 4-fluoro-7-nitro-2,1,3-benzoxadiazole (NBD-F), the D-amino acids were determined by the 2D-HPLC system with fluorescence detectors. As the first dimension, a microbore-monolithic-ODS column (750 mm x 0.53 mm I.D.) was adopted and a self-packed narrowbore-Pirkle type enantioselective column (Sumichiral OA-2500S, 250 mm x 1.5 mm I.D.) was selected for the second dimension. The lower limits of quantitation of D-Ser and D-Ala were 500 amol, and the within-day and day-to-day precisions were less than 6.8%. Using these methods, the amounts of D-Ser and D-Ala in 6 brain tissues, 4 peripheral tissues, serum and urine of mice having various DAO activities were determined; the amounts of these D-amino acids were drastically increased with a lowering of the DAO activity except for the cases of D-Ser in the frontal brain regions. The present micro-2D-HPLC procedures are powerful tools for the determination of small amounts of D-Ser and D-Ala in mammalian samples, and the obtained results would be useful for developing novel drugs that modulate the DAO activity, such as DAO inhibitors, against neuronal diseases.

D-amino acid oxidase: physiological role and applications

D-Amino acids play a key role in regulation of many processes in living cells. FAD-dependent D-amino acid oxidase (DAAO) is one of the most important enzymes responsible for maintenance proper level of D-amino acids. The most interesting and important data for regulation of the nervous system, hormone secretion, and other processes by D-amino acids as well as development of different diseases under changed DAAO activity are presented. The mechanism of regulation is complex and multi-parametric because the same enzyme simultaneously influences the level of different D-amino acids, which can result in opposing effects. Use of DAAO for diagnostic and therapeutic purposes is also considered.

Improved analytical sensitivity reveals the occurrence of gender-related variability in diamine oxidase enzyme activity in healthy individuals

OBJECTIVES

Serum diamine oxidase (DAO; EC 1.4.3.6) activity is often employed as a clinical indicator of the integrity of intestinal mucosa. However, interindividual variation in enzyme activity and reference values for healthy women and men have not been studied.

DESIGN AND METHODS

DAO activity was measured by using cadaverine coupled to O-dianisidine oxidation in 50 healthy individuals.

RESULTS

The mean activity was 7.59+/-3.67 U/L for women and 2.38+/-0.71 U/L for men (p<0.001).

CONCLUSIONS

Gender is a major determinant for DAO activity in healthy subjects.

Reviewing the role of the genes G72 and DAAO in glutamate neurotransmission in schizophrenia

We review the role of two susceptibility genes; G72 and DAAO in glutamate neurotransmission and the aetiology of schizophrenia. The gene product of G72 is an activator of DAAO (D-amino acid oxidase), which is the only enzyme oxidising D-serine. D-serine is an important co-agonist for the NMDA glutamate receptor and plays a role in neuronal migration and cell death. Studies of D-serine revealed lower serum levels in schizophrenia patients as compared to healthy controls. Furthermore, administration of D-serine as add-on medication reduced the symptoms of schizophrenia. The underlying mechanism of the involvement of G72 and DAAO in schizophrenia is probably based on decreased levels of D-serine and decreased NMDA receptor functioning in patients. The involvement of this gene is therefore indirect support for the glutamate dysfunction hypothesis in schizophrenia.

Sensitive Two-Dimensional Determination of Small Amounts of D-Amino Acids in Mammals and the Study on Their Functions

D-amino acids are the candidates of novel physiologically active substances and the marker molecules of diseases in mammals. In the present study, the two-dimensional determination of small amounts of D-amino acids in mammals has been performed after sensitive pre-column fluorescence derivatization with 4-fluoro-7-nitro-2,1,3-benzoxadiazole (NBD-F). The two-dimensional HPLC system includes the isolation of the target amino acids as D+L mixtures using the micro-ODS column, and the determination of the enantiomers using the chiral column. This method enables the sensitive and selective determination of small amounts of D-amino acids in mammals without the interference of L-amino acids and peptides, and the presence and distribution of D-Leu, D-Ala, D-Pro, D-Thr and D-allo-Thr has been demonstrated in rats and mice. The regulation and the origins of these D-amino acids, and their relationships to the biological rhythms are also discussed.

Caenorhabditis eleganshas two genes encoding functionald-aspartate oxidases

Four cDNA clones that were annotated in the database as encoding d-amino acid oxidase (DAAO) or d-aspartate oxidase (DASPO) were isolated by RT-PCR from Caenorhabditis elegans RNA. The proteins (Y69Ap, C47Ap, F18Ep, and F20Hp) encoded by the cloned cDNAs were expressed in Escherichia coli as recombinant proteins with an N-terminal His-tag. All proteins except F20Hp were recovered in the soluble fractions. The recombinant Y69Ap has functional DAAO activity, as it can deaminate neutral and basic d-amino acids, whereas the recombinants C47Ap and F18Ep have functional DASPO activities, as they can deaminate acidic d-amino acids. Additional experiments using purified recombinant proteins revealed that Y69Ap deaminates d-Arg more efficiently than d-Ala and d-Met, and that C47Ap and F18Ep show distinct kinetic properties against d-Asp, d-Glu, and N-methyl-d-Asp. This is the first time that cDNA cloning of invertebrate DAAO and DASPO genes has been reported. In addition, our study reveals for the first time that C. elegans has at least two genes encoding functional DASPOs and one gene encoding DAAO, although it had previously been thought that organisms only bear one copy each of these genes. The two C. elegans DASPOs differ in their substrate specificities and possibly also in their subcellular localization.