A physiological mechanism to regulate d-aspartic acid and NMDA levels in mammals revealed by d-aspartate oxidase deficient mice

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

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

Improvement of Learning and Memory by Elevating Brain D-Aspartate in a Mouse Model of Fragile X Syndrome

Fragile X syndrome (FXS) is an inherited human mental retardation that arises from expansion of a CGG repeat in the Fmr1 gene, causing loss of the fragile X mental retardation protein (FMRP). It is reported that N-methyl-D-aspartate receptor (NMDAR)-mediated facilitation of long-term potentiation (LTP) and fear memory are impaired in Fmr1 knockout (KO) mice. In this study, biological, pharmacological, and electrophysiological techniques were performed to determine the roles of D-aspartate (D-Asp), a modulator of NMDAR, and its metabolizing enzyme D-aspartate oxidase (DDO) in Fmr1 KO mice. Levels of D-Asp were decreased in the medial prefrontal cortex (mPFC ); however, the levels of its metabolizing enzyme DDO were increased. Electrophysiological recordings indicated that oral drinking of D-Asp recovered LTP induction in mPFC from Fmr1 KO mice. Moreover, chronic oral administration of D-Asp reversed behavioral deficits of cognition and locomotor coordination in Fmr1 KO mice. The therapeutic action of D-Asp was partially through regulating functions of NMDARs and mGluR5/mTOR/4E-BP signaling pathways. In conclusion, supplement of D-Asp may benefit for synaptic plasticity and behaviors in Fmr1 KO mice and offer a potential therapeutic strategy for FXS.

Mouse Models to Study Peroxisomal Functions and Disorders: Overview, Caveats, and Recommendations

During the last three decades many mouse lines were created or identified that are deficient in one or more peroxisomal functions. Different methodologies were applied to obtain global, hypomorph, cell type selective, inducible, and knockin mice. Whereas some models closely mimic pathologies in patients, others strongly deviate or no human counterpart has been reported. Often, mice, apparently endowed with a stronger transcriptional adaptation, have to be challenged with dietary additions or restrictions in order to trigger phenotypic changes. Depending on the inactivated peroxisomal protein, several approaches can be taken to validate the loss-of-function. Here, an overview is given of the available mouse models and their most important characteristics.

D-aspartate oxidase gene duplication induces social recognition memory deficit in mice and intellectual disabilities in humans

The D-aspartate oxidase (DDO) gene encodes the enzyme responsible for the catabolism of D-aspartate, an atypical amino acid enriched in the mammalian brain and acting as an endogenous NMDA receptor agonist. Considering the key role of NMDA receptors in neurodevelopmental disorders, recent findings suggest a link between D-aspartate dysmetabolism and schizophrenia. To clarify the role of D-aspartate on brain development and functioning, we used a mouse model with constitutive Ddo overexpression and D-aspartate depletion. In these mice, we found reduced number of BrdU-positive dorsal pallium neurons during corticogenesis, and decreased cortical and striatal gray matter volume at adulthood. Brain abnormalities were associated with social recognition memory deficit at juvenile phase, suggesting that early D-aspartate occurrence influences neurodevelopmental related phenotypes. We corroborated this hypothesis by reporting the first clinical case of a young patient with severe intellectual disability, thought disorders and autism spectrum disorder symptomatology, harboring a duplication of a chromosome 6 region, including the entire DDO gene.

d-Amino Acids and pLG72 in Alzheimer's Disease and Schizophrenia

Numerous studies over the last several years have shown that d-amino acids, especially d-serine, have been related to brain and neurological disorders. Acknowledged neurological functions of d-amino acids include neurotransmission and learning and memory functions through modulating N-methyl-d-aspartate type glutamate receptors (NMDARs). Aberrant d-amino acids level and polymorphisms of genes related to d-amino acids metabolism are associated with neurodegenerative brain conditions. This review summarizes the roles of d-amino acids and pLG72, also known as d-amino acid oxidase activator, on two neurodegenerative disorders, schizophrenia and Alzheimer’s disease (AD). The scope includes the changes in d-amino acids levels, gene polymorphisms of G72 genomics, and the role of pLG72 on NMDARs and mitochondria in schizophrenia and AD. The clinical diagnostic value of d-amino acids and pLG72 and the therapeutic importance are also reviewed.

New Evidence on the Role of D-Aspartate Metabolism in Regulating Brain and Endocrine System Physiology: From Preclinical Observations to Clinical Applications

The endogenous amino acids serine and aspartate occur at high concentrations in free D-form in mammalian organs, including the central nervous system and endocrine glands. D-serine (D-Ser) is largely localized in the forebrain structures throughout pre and postnatal life. Pharmacologically, D-Ser plays a functional role by acting as an endogenous coagonist at N-methyl-D-aspartate receptors (NMDARs). Less is known about the role of free D-aspartate (D-Asp) in mammals. Notably, D-Asp has a specific temporal pattern of occurrence. In fact, free D-Asp is abundant during prenatal life and decreases greatly after birth in concomitance with the postnatal onset of D-Asp oxidase expression, which is the only enzyme known to control endogenous levels of this molecule. Conversely, in the endocrine system, D-Asp concentrations enhance after birth during its functional development, thereby suggesting an involvement of the amino acid in the regulation of hormone biosynthesis. The substantial binding affinity for the NMDAR glutamate site has led us to investigate the in vivo implications of D-Asp on NMDAR-mediated responses. Herein we review the physiological function of free D-Asp and of its metabolizing enzyme in regulating the functions of the brain and of the neuroendocrine system based on recent genetic and pharmacological human and animal studies.

d-Aspartate N-methyltransferase catalyzes biosynthesis of N-methyl-d-aspartate (NMDA), a well-known selective agonist of the NMDA receptor, in mice

N-Methyl-d-aspartate (NMDA), which is a selective agonist for the NMDA receptor, has recently been shown to be present in various biological tissues. In mammals, the activity of d-aspartate N-methyltransferase (DDNMT), which produces NMDA from d-aspartate, has been detected only in homogenates prepared from rat tissues. Moreover, the enzymatic properties of DDNMT have been poorly studied and its molecular entity has not yet been identified. In this report, we show for the first time that the activity of DDNMT is present in mouse tissues and succeed in obtaining a partially purified enzyme preparation from a mouse tissue homogenate with a purification fold of 1900 or more, and have characterized the enzymatic activity of this preparation. The results indicate that DDNMT, which is highly specific for d-aspartate and is S-adenosyl-l-methionine-dependent, is a novel enzyme that clearly differs from the known methylamine-glutamate N-methyltransferase (EC 2.1.1.21) and glycine N-methyltransferase (EC 2.1.1.20).

Dysfunctional d-aspartate metabolism in BTBR mouse model of idiopathic autism

BACKGROUND

Autism spectrum disorders (ASD) comprise a heterogeneous group of neurodevelopmental conditions characterized by impairment in social interaction, deviance in communication, and repetitive behaviors. Dysfunctional ionotropic NMDA and AMPA receptors, and metabotropic glutamate receptor 5 activity at excitatory synapses has been recently linked to multiple forms of ASD. Despite emerging evidence showing that d-aspartate and d-serine are important neuromodulators of glutamatergic transmission, no systematic investigation on the occurrence of these D-amino acids in preclinical ASD models has been carried out.

METHODS

Through HPLC and qPCR analyses we investigated d-aspartate and d-serine metabolism in the brain and serum of four ASD mouse models. These include BTBR mice, an idiopathic model of ASD, and Cntnap2^-/-, Shank3^-/-, and 16p11.2^+/- mice, three established genetic mouse lines recapitulating high confidence ASD-associated mutations.

RESULTS

Biochemical and gene expression mapping in Cntnap2^-/-, Shank3^-/-, and 16p11.2^+/- failed to find gross cerebral and serum alterations in d-aspartate and d-serine metabolism. Conversely, we found a striking and stereoselective increased d-aspartate content in the prefrontal cortex, hippocampus and serum of inbred BTBR mice. Consistent with biochemical assessments, in the same brain areas we also found a robust reduction in mRNA levels of d-aspartate oxidase, encoding the enzyme responsible for d-aspartate catabolism.

CONCLUSIONS

Our results demonstrated the presence of disrupted d-aspartate metabolism in a widely used animal model of idiopathic ASD.

GENERAL SIGNIFICANCE

Overall, this work calls for a deeper investigation of D-amino acids in the etiopathology of ASD and related developmental disorders.

New insights on the influence of free d-aspartate metabolism in the mammalian brain during prenatal and postnatal life

Free d-aspartate is abundant in the mammalian embryonic brain. However, following the postnatal onset of the catabolic d-aspartate oxidase (DDO) activity, cerebral d-aspartate levels drastically decrease, remaining constantly low throughout life. d-Aspartate stimulates both glutamatergic NMDA receptors (NMDARs) and metabotropic Glu5 receptors. In rodents, short-term d-aspartate exposure increases spine density and synaptic plasticity, and improves cognition. Conversely, persistently high d-Asp levels produce NMDAR-dependent neurotoxic effects, leading to precocious neuroinflammation and cell death. These pieces of evidence highlight the dichotomous impact of d-aspartate signaling on NMDAR-dependent processes and, in turn, unveil a neuroprotective role for DDO in preventing the detrimental effects of excessive d-aspartate stimulation during aging. Here, we will focus on the in vivo influence of altered d-aspartate metabolism on the modulation of glutamatergic functions and its involvement in translational studies. Finally, preliminary data on the role of embryonic d-aspartate in the mouse brain will also be reviewed.

D-Aspartic Acid in Vertebrate Reproduction: Animal Models and Experimental Designs‡

This article reviews the animal models and experimental designs that have been used during the past twenty years to demonstrate the prominent role played by d-aspartate (d-Asp) in the reproduction of vertebrates, from amphibians to humans. We have tabulated the findings of in vivo and in vitro experiments that demonstrate the effects of d-Asp uptake on hormone production and gametogenesis in vertebrate animal models. The contribution of each animal model to the existing knowledge on the role of d-Asp in reproductive processes has been discussed. A critical analysis of experimental designs has also been carried out. Experiments performed on wild animal species suggest a role of d-Asp in the mechanisms that regulate the reproductive cycle. Several in vivo and in vitro studies carried out on mouse and rat models have facilitated an understanding of the molecular pathways activated by D-Asp in both steroidogenesis and spermatogenesis, with particular emphasis on testosterone biosynthesis. Some attempts using d-Asp for the improvement of reproductive activity in animals of commercial interest have yielded mixed results. The increased transcriptome activity of enzymes and receptors involved in the reproductive activity in d-Asp-treated broiler roosters revealed further details on the mechanism of action of d-Asp on the reproductive processes. The close relationship between d-Asp and reproductive activity has emerged, particularly in relation to its effects exerted on semen quality, proposing therapeutic applications of this amino acid in andrology and in medically-assisted procreation techniques.

Secreted d-aspartate oxidase functions in C. elegans reproduction and development

d-Aspartate oxidase (DDO) is a degradative enzyme that acts stereospecifically on free acidic D-amino acids such as d-aspartate and d-glutamate. d-Aspartate plays an important role in regulating neurotransmission, developmental processes, hormone secretion, and reproductive functions in mammals. In contrast, the physiological role of d-glutamate in mammals remains unclear. In Caenorhabditis elegans, the enzyme responsible for in vivo metabolism of d-glutamate is DDO-3, one of the three DDO isoforms, which is also required for normal self-fertility, hatching, and lifespan. In general, eukaryotic DDOs localize to subcellular peroxisomes in a peroxisomal targeting signal type 1 (PTS1)-dependent manner. However, DDO-3 does not contain a PTS1, but instead has a putative N-terminal signal peptide (SP). In this study, we found that DDO-3 is a secreted DDO, the first such enzyme to be described in eukaryotes. In hermaphrodites, DDO-3 was secreted from the proximal gonadal sheath cells in a SP-dependent manner and transferred to the oocyte surface. In males, DDO-3 was secreted from the seminal vesicle into the seminal fluid in a SP-dependent manner during mating with hermaphrodites. In both sexes, DDO-3 was secreted from the cells where it was produced into the body fluid and taken up by scavenger coelomocytes. Full-length DDO-3 transgene rescued all phenotypes elicited by the deletion of ddo-3, whereas a DDO-3 transgene lacking the putative SP did not. Together, these results indicate that secretion of DDO-3 is essential for its physiological functions.

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

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

The Emerging Role of Altered d-Aspartate Metabolism in Schizophrenia: New Insights From Preclinical Models and Human Studies

Besides d-serine, another d-amino acid with endogenous occurrence in the mammalian brain, d-aspartate, has been recently shown to influence NMDA receptor (NMDAR)-mediated transmission. d-aspartate is present in the brain at extracellular level in nanomolar concentrations, binds to the agonist site of NMDARs and activates this subclass of glutamate receptors. Along with its direct effect on NMDARs, d-aspartate can also evoke considerable l-glutamate release in specific brain areas through the presynaptic activation of NMDA, AMPA/kainate and mGlu5 receptors. d-aspartate is enriched in the embryonic brain of rodents and humans and its concentration strongly decreases after birth, due to the post-natal expression of the catabolising enzyme d-aspartate oxidase (DDO). Based on the hypothesis of NMDAR hypofunction in schizophrenia pathogenesis, recent preclinical and clinical studies suggested a relationship between perturbation of d-aspartate metabolism and this psychiatric disorder. Consistently, neurophysiological and behavioral characterization of Ddo knockout (Ddo ^-/-) and d-aspartate-treated mice highlighted that abnormally higher endogenous d-aspartate levels significantly increase NMDAR-mediated synaptic plasticity, neuronal spine density and memory. Remarkably, increased d-aspartate levels influence schizophrenia-like phenotypes in rodents, as indicated by improved fronto-hippocampal connectivity, attenuated prepulse inhibition deficits and reduced activation of neuronal circuitry induced by phencyclidine exposure. In healthy humans, a genetic polymorphism associated with reduced prefrontal DDO gene expression predicts changes in prefrontal phenotypes including greater gray matter volume and enhanced functional activity during working memory. Moreover, neurochemical detections in post-mortem brain of schizophrenia-affected patients have shown significantly reduced d-aspartate content in prefrontal regions, associated with increased DDO mRNA expression or DDO enzymatic activity. Overall, these findings suggest a possible involvement of dysregulated embryonic d-aspartate metabolism in schizophrenia pathophysiology and, in turn, highlight the potential use of free d-aspartate supplementation as a new add-on therapy for treating the cognitive symptoms of this mental illness.

Chiral Measurement of Aspartate and Glutamate in Single Neurons by Large-Volume Sample Stacking Capillary Electrophoresis

d-Amino acids (d-AAs) are endogenous molecules found throughout the metazoan, the functions of which remain poorly understood. Measurements of low abundance and heterogeneously distributed d-AAs in complex biological samples, such as cells and multicellular structures of the central nervous system (CNS), require the implementation of sensitive and selective analytical approaches. In order to measure the d- and l-forms of aspartate and glutamate, we developed and applied a stacking chiral capillary electrophoresis (CE) with laser-induced fluorescence detection method. The achieved online analyte preconcentration led to a 480-fold enhancement of detection sensitivity relative to capillary zone electrophoresis, without impacting separation resolution or analysis time. Additionally, the effects of inorganic ions on sample preconcentration and CE separation were evaluated. The approach enabled the relative quantification of d-aspartate and d-glutamate in individual neurons mechanically isolated from the CNS of the sea slug Aplysia californica, a well characterized neurobiological model. Levels of these structurally similar d-AAs were significantly different in subpopulations of cells collected from the investigated neuronal clusters.

An overview on D-amino acids

More than half a century ago researchers thought that D-amino acids had a minor function compared to L-enantiomers in biological processes. Many evidences have shown that D-amino acids are present in high concentration in microorganisms, plants, mammals and humans and fulfil specific biological functions. In the brain of mammals, D-serine (D-Ser) acts as a co-agonist of the N-methyl-D-aspartate (NMDA)-type glutamate receptors, responsible for learning, memory and behaviour. D-Ser metabolism is relevant for disorders associated with an altered function of the NMDA receptor, such as schizophrenia, ischemia, epilepsy and neurodegenerative disorders. On the other hand, D-aspartate (D-Asp) is one of the major regulators of adult neurogenesis and plays an important role in the development of endocrine function. D-Asp is present in the neuroendocrine and endocrine tissues and testes, and regulates the synthesis and secretion of hormones and spermatogenesis. Also food proteins contain D-amino acids that are naturally originated or processing-induced under conditions such as high temperatures, acid and alkali treatments and fermentation processes. The presence of D-amino acids in dairy products denotes thermal and alkaline treatments and microbial contamination. Two enzymes are involved in the metabolism of D-amino acids: amino acid racemase in the synthesis and D-amino acid oxidase in the degradation.

Quantitative determination of free D-Asp, L-Asp and N-methyl-D-aspartate in mouse brain tissues by chiral separation and Multiple Reaction Monitoring tandem mass spectrometry

Several studies have suggested that free d-Asp has a crucial role in N-methyl d-Asp receptor-mediated neurotransmission playing very important functions in physiological and pathological processes. This paper describes the development of an analytical procedure for the direct and simultaneous determination of free d-Asp, l-Asp and N-methyl d-Asp in specimens of different mouse brain tissues using chiral LC-MS/MS in Multiple Reaction Monitoring scan mode. After comparing three procedures and different buffers and extraction solvents, a simple preparation procedure was selected the analytes of extraction. The method was validated by analyzing l-Asp, d-Asp and N-methyl d-Asp recovery at different spiked concentrations (50, 100 and 200 pg/μl) yielding satisfactory recoveries (75–110%), and good repeatability. Limits of detection (LOD) resulted to be 0.52 pg/μl for d-Asp, 0.46 pg/μl for l-Asp and 0.54 pg/μl for NMDA, respectively. Limits of quantification (LOQ) were 1.57 pg/μl for d-Asp, 1.41 pg/μl for l-Asp and 1.64 pg/μl for NMDA, respectively. Different concentration levels were used for constructing the calibration curves which showed good linearity. The validated method was then successfully applied to the simultaneous detection of d-Asp, l-Asp and NMDA in mouse brain tissues. The concurrent, sensitive, fast, and reproducible measurement of these metabolites in brain tissues will be useful to correlate the amount of free d-Asp with relevant neurological processes, making the LC-MS/MS MRM method well suited, not only for research work but also for clinical analyses.

Structure-function relationships in human d-aspartate oxidase: characterisation of variants corresponding to known single nucleotide polymorphisms

d-Aspartate oxidase (DDO) is a degradative enzyme that is stereospecific for the acidic amino acid d-aspartate, an endogenous agonist of the N-methyl-d-aspartate (NMDA) receptor. Dysregulation of NMDA receptor-mediated neurotransmission has been implicated in the onset of various neuropsychiatric disorders including schizophrenia and in chronic pain. Thus, appropriate regulation of the amount of d-aspartate is believed to be important for maintaining proper neural activity in the nervous system. Herein, the effects of the non-synonymous single nucleotide polymorphisms (SNPs) R216Q and S308N on several properties of human DDO were examined. Analysis of the purified recombinant enzyme showed that the R216Q and S308N substitutions reduce enzyme activity towards acidic d-amino acids, decrease the binding affinity for the coenzyme flavin adenine dinucleotide and decrease the temperature stability. Consistent with these findings, further experiments using cultured mammalian cells revealed elevated d-aspartate in cultures of R216Q and S308N cells compared with cells expressing wild-type DDO. Furthermore, accumulation of several amino acids other than d-aspartate also differed between these cultures. Thus, expression of DDO genes carrying the R216Q or S308N SNP substitutions may increase the d-aspartate content in humans and alter homeostasis of several other amino acids. This work may aid in understanding the correlation between DDO activity and the risk of onset of NMDA receptor-related diseases.

Olanzapine, but not clozapine, increases glutamate release in the prefrontal cortex of freely moving mice by inhibiting D-aspartate oxidase activity

D-aspartate levels in the brain are regulated by the catabolic enzyme D-aspartate oxidase (DDO). D-aspartate activates NMDA receptors, and influences brain connectivity and behaviors relevant to schizophrenia in animal models. In addition, recent evidence reported a significant reduction of D-aspartate levels in the post-mortem brain of schizophrenia-affected patients, associated to higher DDO activity. In the present work, microdialysis experiments in freely moving mice revealed that exogenously administered D-aspartate efficiently cross the blood brain barrier and stimulates L-glutamate efflux in the prefrontal cortex (PFC). Consistently, D-aspartate was able to evoke L-glutamate release in a preparation of cortical synaptosomes through presynaptic stimulation of NMDA, mGlu5 and AMPA/kainate receptors. In support of a potential therapeutic relevance of D-aspartate metabolism in schizophrenia, in vitro enzymatic assays revealed that the second-generation antipsychotic olanzapine, differently to clozapine, chlorpromazine, haloperidol, bupropion, fluoxetine and amitriptyline, inhibits the human DDO activity. In line with in vitro evidence, chronic systemic administration of olanzapine induces a significant extracellular release of D-aspartate and L-glutamate in the PFC of freely moving mice, which is suppressed in Ddo knockout animals. These results suggest that the second-generation antipsychotic olanzapine, through the inhibition of DDO activity, increases L-glutamate release in the PFC of treated mice.

Age-Related Changes in D-Aspartate Oxidase Promoter Methylation Control Extracellular D-Aspartate Levels and Prevent Precocious Cell Death during Brain Aging

The endogenous NMDA receptor (NMDAR) agonist D-aspartate occurs transiently in the mammalian brain because it is abundant during embryonic and perinatal phases before drastically decreasing during adulthood. It is well established that postnatal reduction of cerebral D-aspartate levels is due to the concomitant onset of D-aspartate oxidase (DDO) activity, a flavoenzyme that selectively degrades bicarboxylic D-amino acids. In the present work, we show that d-aspartate content in the mouse brain drastically decreases after birth, whereas Ddo mRNA levels concomitantly increase. Interestingly, postnatal Ddo gene expression is paralleled by progressive demethylation within its putative promoter region. Consistent with an epigenetic control on Ddo expression, treatment with the DNA-demethylating agent, azacitidine, causes increased mRNA levels in embryonic cortical neurons. To indirectly evaluate the effect of a putative persistent Ddo gene hypermethylation in the brain, we used Ddo knock-out mice (Ddo(-/-)), which show constitutively suppressed Ddo expression. In these mice, we found for the first time substantially increased extracellular content of d-aspartate in the brain. In line with detrimental effects produced by NMDAR overstimulation, persistent elevation of D-aspartate levels in Ddo(-/-) brains is associated with appearance of dystrophic microglia, precocious caspase-3 activation, and cell death in cortical pyramidal neurons and dopaminergic neurons of the substantia nigra pars compacta. This evidence, along with the early accumulation of lipufuscin granules in Ddo(-/-) brains, highlights an unexpected importance of Ddo demethylation in preventing neurodegenerative processes produced by nonphysiological extracellular levels of free D-aspartate.

D-Amino Acids in the Nervous and Endocrine Systems

Amino acids are important components for peptides and proteins and act as signal transmitters. Only L-amino acids have been considered necessary in mammals, including humans. However, diverse D-amino acids, such as D-serine, D-aspartate, D-alanine, and D-cysteine, are found in mammals. Physiological roles of these D-amino acids not only in the nervous system but also in the endocrine system are being gradually revealed. N-Methyl-D-aspartate (NMDA) receptors are associated with learning and memory. D-Serine, D-aspartate, and D-alanine can all bind to NMDA receptors. H₂S generated from D-cysteine reduces disulfide bonds in receptors and potentiates their activity. Aberrant receptor activity is related to diseases of the central nervous system (CNS), such as Alzheimer's disease, amyotrophic lateral sclerosis, and schizophrenia. Furthermore, D-amino acids are detected in parts of the endocrine system, such as the pineal gland, hypothalamus, pituitary gland, pancreas, adrenal gland, and testis. D-Aspartate is being investigated for the regulation of hormone release from various endocrine organs. Here we focused on recent findings regarding the synthesis and physiological functions of D-amino acids in the nervous and endocrine systems.

D-aspartate dysregulation in Ddo(-/-) mice modulates phencyclidine-induced gene expression changes of postsynaptic density molecules in cortex and striatum

N-methyl-D-aspartate receptor (NMDAR) hypofunction has been considered a key alteration in schizophrenia pathophysiology. Thus, several strategies aimed at enhancing glutamatergic transmission, included the introduction in therapy of D-amino acids, such as D-serine and D-cycloserine augmentation, have been proposed to counteract difficult-to-treat symptoms or treatment-resistant forms of schizophrenia. Another D-amino acid, D-aspartate, has recently gained increasing interest for its role in NMDAR activation and has been found reduced in post-mortem cortex of schizophrenia patients. NMDAR is the core of the postsynaptic density (PSD), a postsynaptic site involved in glutamate signaling and responsive to antipsychotic treatment. In this study, we investigated striatal and cortical gene expression of key PSD transcripts (i.e. Homer1a, Homer1b/c, and PSD-95) in mice with persistently elevated brain D-aspartate-levels, i.e. the D-aspartate-oxidase knockout mice (Ddo(-/-)). These animal models were analyzed both in naive condition and after phencyclidine (PCP) treatment. Naive Ddo(-/-) mice showed decreased Homer1a expression in the prefrontal cortex, increased Homer1b/c expression in the striatum, and decreased PSD-95 expression in the striatum and in the cortex. Acute PCP treatment restored, and even potentiated, Homer1a expression in the prefrontal cortex of mutant mice, while it had limited effects on the other genes. These results suggest that persistently elevated D-aspartate, by enhancing NMDA transmission, may cause complex adaptive mechanisms affecting Homer1a, which in turn may explain the recently demonstrated protective effects of this D-amino acid against PCP-induced behavioral alterations, such as ataxic behavior.

A role for D-aspartate oxidase in schizophrenia and in schizophrenia-related symptoms induced by phencyclidine in mice

Increasing evidence points to a role for dysfunctional glutamate N-methyl-D-aspartate receptor (NMDAR) neurotransmission in schizophrenia. D-aspartate is an atypical amino acid that activates NMDARs through binding to the glutamate site on GluN2 subunits. D-aspartate is present in high amounts in the embryonic brain of mammals and rapidly decreases after birth, due to the activity of the enzyme D-aspartate oxidase (DDO). The agonistic activity exerted by D-aspartate on NMDARs and its neurodevelopmental occurrence make this D-amino acid a potential mediator for some of the NMDAR-related alterations observed in schizophrenia. Consistently, substantial reductions of D-aspartate and NMDA were recently observed in the postmortem prefrontal cortex of schizophrenic patients. Here we show that DDO mRNA expression is increased in prefrontal samples of schizophrenic patients, thus suggesting a plausible molecular event responsible for the D-aspartate imbalance previously described. To investigate whether altered D-aspartate levels can modulate schizophrenia-relevant circuits and behaviors, we also measured the psychotomimetic effects produced by the NMDAR antagonist, phencyclidine, in Ddo knockout mice (Ddo(-)(/-)), an animal model characterized by tonically increased D-aspartate levels since perinatal life. We show that Ddo(-/-) mice display a significant reduction in motor hyperactivity and prepulse inhibition deficit induced by phencyclidine, compared with controls. Furthermore, we reveal that increased levels of D-aspartate in Ddo(-/-) animals can significantly inhibit functional circuits activated by phencyclidine, and affect the development of cortico-hippocampal connectivity networks potentially involved in schizophrenia. Collectively, the present results suggest that altered D-aspartate levels can influence neurodevelopmental brain processes relevant to schizophrenia.

D-aspartate modulates nociceptive-specific neuron activity and pain threshold in inflammatory and neuropathic pain condition in mice

D-Aspartate (D-Asp) is a free D-amino acid found in the mammalian brain with a temporal-dependent concentration based on the postnatal expression of its metabolizing enzyme D-aspartate oxidase (DDO). D-Asp acts as an agonist on NMDA receptors (NMDARs). Accordingly, high levels of D-Asp in knockout mice for Ddo gene (Ddo (-/-)) or in mice treated with D-Asp increase NMDAR-dependent processes. We have here evaluated in Ddo (-/-) mice the effect of high levels of free D-Asp on the long-term plastic changes along the nociceptive pathway occurring in chronic and acute pain condition. We found that Ddo (-/-) mice show an increased evoked activity of the nociceptive specific (NS) neurons of the dorsal horn of the spinal cord (L4-L6) and a significant decrease of mechanical and thermal thresholds, as compared to control mice. Moreover, Ddo gene deletion exacerbated the nocifensive responses in the formalin test and slightly reduced pain thresholds in neuropathic mice up to 7 days after chronic constriction injury. These findings suggest that the NMDAR agonist, D-Asp, may play a role in the regulation of NS neuron electrophysiological activity and behavioral responses in physiological and pathological pain conditions.

Free D-aspartate regulates neuronal dendritic morphology, synaptic plasticity, gray matter volume and brain activity in mammals

D-aspartate (D-Asp) is an atypical amino acid, which is especially abundant in the developing mammalian brain, and can bind to and activate N-methyl-D-Aspartate receptors (NMDARs). In line with its pharmacological features, we find that mice chronically treated with D-Asp show enhanced NMDAR-mediated miniature excitatory postsynaptic currents and basal cerebral blood volume in fronto-hippocampal areas. In addition, we show that both chronic administration of D-Asp and deletion of the gene coding for the catabolic enzyme D-aspartate oxidase (DDO) trigger plastic modifications of neuronal cytoarchitecture in the prefrontal cortex and CA1 subfield of the hippocampus and promote a cytochalasin D-sensitive form of synaptic plasticity in adult mouse brains. To translate these findings in humans and consistent with the experiments using Ddo gene targeting in animals, we performed a hierarchical stepwise translational genetic approach. Specifically, we investigated the association of variation in the gene coding for DDO with complex human prefrontal phenotypes. We demonstrate that genetic variation predicting reduced expression of DDO in postmortem human prefrontal cortex is mapped on greater prefrontal gray matter and activity during working memory as measured with MRI. In conclusion our results identify novel NMDAR-dependent effects of D-Asp on plasticity and physiology in rodents, which also map to prefrontal phenotypes in humans.

Decreased levels of D-aspartate and NMDA in the prefrontal cortex and striatum of patients with schizophrenia

The potential implication of a decrease in the function of N-methyl-d-aspartate receptors (NMDARs) in the pathophysiology of schizophrenia has long been hypothesised. Accordingly, compounds that inhibit the glycine-1 transporter or target the glycine-binding site of NMDARs, including the co-agonists D-serine and glycine, have shown promise in treating the symptoms of schizophrenia. Clinical interest for d-serine has also been supported by evidence for its abnormal metabolism in schizophrenic patients. Together with D-serine, another D-form amino acid, D-aspartate, exists in the brain of mammals. Synthesised by the enzyme aspartate racemase, D-aspartate is highly concentrated in the prenatal brain; after birth, its levels sharply decrease due to the catabolising activity of the enzyme D-aspartate oxidase. D-aspartate is able to stimulate NMDAR-dependent neurotransmission through direct action at the glutamate-binding site of NMDARs, thus functioning as an endogenous agonist for this subclass of glutamate receptors. In this study, we evaluated for the first time the content of D-aspartate and of its derivative, NMDA, in the post-mortem prefrontal cortex and striatum of schizophrenic patients. Moreover, in the same brain samples, we analysed the expression levels of the subunits that form NMDARs, which are the in vivo targets of D-aspartate and NMDA. Interestingly, we found that D-aspartate and NMDA are consistently decreased in schizophrenia brains compared to control brains. In the prefrontal cortex, this decrease is correlated with a marked downregulation of NMDAR subunits. Overall, these results agree with the innovative therapeutic research in schizophrenia that is aimed at targeting glutamatergic transmission via D-amino acids.

Abnormal NMDA receptor function exacerbates experimental autoimmune encephalomyelitis

BACKGROUND AND PURPOSE

Glutamate transmission is dysregulated in both multiple sclerosis (MS) and experimental autoimmune encephalomyelitis (EAE), the animal model of MS. A characteristic of EAE is increased glutamate transmission associated with up-regulation of AMPA receptors. However, little is known about the role of NMDA receptors in the synaptic modifications induced by EAE.

EXPERIMENTAL APPROACH

The contribution of NMDA receptors to the alterations of glutamate transmission and disease severity in EAE mice was assessed by means of neurophysiological, morphological, Western blot, metabolic and clinical score assessments.

KEY RESULTS

In our EAE mice, there was an NMDA receptor-dependent increase of glutamate release, associated with marked activation of the astroglia. Presynaptic NMDA receptors became overactive during EAE, increasing synaptic glutamate release by a mechanism dependent on voltage-gated sodium channels. By means of NAD(P)H autofluorescence analysis, we also found that EAE has a glutamate and NMDA receptor-dependent dysfunction of mitochondrial activity, which is known to contribute to the neurodegenerative damage of MS and EAE. Furthermore, pharmacological blockade of NMDA receptors in vivo ameliorated both synaptic transmission defects and of the clinical disease course of EAE mice, while EAE induced in mice with a genetically enhanced NMDA receptor signalling had opposite effects.

CONCLUSIONS AND IMPLICATIONS

Our data, showing both sensitization of NMDA receptors and their involvement in the progression of the EAE disease, supggest that pharmacological impairment of NMDA receptor signalling would be a component of a neuroprotection strategy in MS.

New insights on the role of free D-aspartate in the mammalian brain

Free D-aspartate (D-Asp) occurs in substantial amounts in the brain at the embryonic phase and in the first few postnatal days, and strongly decreases in adulthood. Temporal reduction of D-Asp levels depends on the postnatal onset of D-aspartate oxidase (DDO) activity, the only enzyme able to selectively degrade this D-amino acid. Several results indicate that D-Asp binds and activates N-methyl-D-aspartate receptors (NMDARs). Accordingly, recent studies have demonstrated that deregulated, higher levels of D-Asp, in knockout mice for Ddo gene and in D-Asp-treated mice, modulate hippocampal NMDAR-dependent long-term potentiation (LTP) and spatial memory. Moreover, similarly to D-serine, administration of D-Asp to old mice is able to rescue the physiological age-related decay of hippocampal LTP. In agreement with a neuromodulatory action of D-Asp on NMDARs, increased levels of this D-amino acid completely suppress long-term depression at corticostriatal synapses and attenuate the prepulse inhibition deficits produced in mice by the psychotomimetic drugs, amphetamine and MK-801. Based on the evidence which points to the ability of D-Asp to act as an endogenous agonist on NMDARs and considering the abundance of D-Asp during prenatal and early life, future studies will be crucial to address the effect of this molecule in the developmental processes of the brain controlled by the activation of NMDARs.

Corticostriatal Plastic Changes in Experimental L-DOPA-Induced Dyskinesia

In Parkinson's disease (PD), alteration of dopamine- (DA-) dependent striatal functions and pulsatile stimulation of DA receptors caused by the discontinuous administration of levodopa (L-DOPA) lead to a complex cascade of events affecting the postsynaptic striatal neurons that might account for the appearance of L-DOPA-induced dyskinesia (LID). Experimental models of LID have been widely used and extensively characterized in rodents and electrophysiological studies provided remarkable insights into the inner mechanisms underlying L-DOPA-induced corticostriatal plastic changes. Here we provide an overview of recent findings that represent a further step into the comprehension of mechanisms underlying maladaptive changes of basal ganglia functions in response to L-DOPA and associated to development of LID.

Nutritional and medicinal aspects of D-amino acids

This paper reviews and interprets a method for determining the nutritional value of D-amino acids, D-peptides, and amino acid derivatives using a growth assay in mice fed a synthetic all-amino acid diet. A large number of experiments were carried out in which a molar equivalent of the test compound replaced a nutritionally essential amino acid such as L-lysine (L-Lys), L-methionine (L-Met), L-phenylalanine (L-Phe), and L-tryptophan (L-Trp) as well as the semi-essential amino acids L-cysteine (L-Cys) and L-tyrosine (L-Tyr). The results show wide-ranging variations in the biological utilization of test substances. The method is generally applicable to the determination of the biological utilization and safety of any amino acid derivative as a potential nutritional source of the corresponding L-amino acid. Because the organism is forced to use the D-amino acid or amino acid derivative as the sole source of the essential or semi-essential amino acid being replaced, and because a free amino acid diet allows better control of composition, the use of all-amino-acid diets for such determinations may be preferable to protein-based diets. Also covered are brief summaries of the widely scattered literature on dietary and pharmacological aspects of 27 individual D-amino acids, D-peptides, and isomeric amino acid derivatives and suggested research needs in each of these areas. The described results provide a valuable record and resource for further progress on the multifaceted aspects of D-amino acids in food and biological samples.

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

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