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

Further evidence supporting the role of GTDC1 in glycine metabolism and neurodevelopmental disorders

Copy number variants (CNVs) represent the genetic cause of about 15-20% of neurodevelopmental disorders (NDDs). We identified a ~67 kb de novo intragenic deletion on chromosome 2q22.3 in a female individual showing a developmental encephalopathy characterised by epilepsy, severe intellectual disability, speech delay, microcephaly, and thin corpus callosum with facial dysmorphisms. The microdeletion involved exons 5-6 of GTDC1, encoding a putative glycosyltransferase, whose expression is particularly enriched in the nervous system. In a previous study, a balanced de novo translocation encompassing GTDC1 was reported in a male child with global developmental delay and delayed speech and language development. Based on these premises, we explored the transcriptomic profile of our proband to evaluate the functional consequences of the novel GTDC1 de novo intragenic deletion in relation to the observed neurodevelopmental phenotype. RNA-seq on the proband's lymphoblastoid cell line (LCL) showed expression changes of glycine/serine and cytokine/chemokine signalling pathways, which are related to neurodevelopment and epileptogenesis. Subsequent analysis by ELISA (enzyme-linked immunosorbent assay) and HPLC (high-performance liquid chromatography) revealed increased levels of glycine in the proband's LCL and serum compared to matched controls. Given that an increased level of glycine has been observed in the plasma samples of individuals with Rett syndrome, a condition sharing epilepsy, microcephaly, and intellectual disability with our proband, we proposed that the GTDC1 downregulation is implicated in neurodevelopmental impairment by altering glycine metabolism. Furthermore, our findings expanded the phenotypic spectrum of the novel GTDC1-related condition, including microcephaly and epilepsy among relevant clinical features.

Decreased free D-aspartate levels in the blood serum of patients with schizophrenia

Introduction

Schizophrenia (SCZ) and autism spectrum disorder (ASD) are neurodevelopmental diseases characterized by different psychopathological manifestations and divergent clinical trajectories. Various alterations at glutamatergic synapses have been reported in both disorders, including abnormal NMDA and metabotropic receptor signaling.

Methods

We conducted a bicentric study to assess the blood serum levels of NMDA receptors-related glutamatergic amino acids and their precursors, including L-glutamate, L-glutamine, D-aspartate, L-aspartate, L-asparagine, D-serine, L-serine and glycine, in ASD, SCZ patients and their respective control subjects. Specifically, the SCZ patients were subdivided into treatment-resistant and non-treatment-resistant SCZ patients, based on their responsivity to conventional antipsychotics.

Results

D-serine and D-aspartate serum reductions were found in SCZ patients compared to controls. Conversely, no significant differences between cases and controls were found in amino acid concentrations in the two ASD cohorts analyzed.

Discussion

This result further encourages future research to evaluate the predictive role of selected D-amino acids as peripheral markers for SCZ pathophysiology and diagnosis.

Asparagine reduces the risk of schizophrenia: a bidirectional two-sample mendelian randomization study of aspartate, asparagine and schizophrenia

BACKGROUND

Despite ongoing research, the underlying causes of schizophrenia remain unclear. Aspartate and asparagine, essential amino acids, have been linked to schizophrenia in recent studies, but their causal relationship is still unclear. This study used a bidirectional two-sample Mendelian randomization (MR) method to explore the causal relationship between aspartate and asparagine with schizophrenia.

METHODS

This study employed summary data from genome-wide association studies (GWAS) conducted on European populations to examine the correlation between aspartate and asparagine with schizophrenia. In order to investigate the causal effects of aspartate and asparagine on schizophrenia, this study conducted a two-sample bidirectional MR analysis using genetic factors as instrumental variables.

RESULTS

No causal relationship was found between aspartate and schizophrenia, with an odds ratio (OR) of 1.221 (95%CI: 0.483-3.088, P-value = 0.674). Reverse MR analysis also indicated that no causal effects were found between schizophrenia and aspartate, with an OR of 0.999 (95%CI: 0.987-1.010, P-value = 0.841). There is a negative causal relationship between asparagine and schizophrenia, with an OR of 0.485 (95%CI: 0.262-0.900, P-value = 0.020). Reverse MR analysis indicates that there is no causal effect between schizophrenia and asparagine, with an OR of 1.005(95%CI: 0.999-1.011, P-value = 0.132).

CONCLUSION

This study suggests that there may be a potential risk reduction for schizophrenia with increased levels of asparagine, while also indicating the absence of a causal link between elevated or diminished levels of asparagine in individuals diagnosed with schizophrenia. There is no potential causal relationship between aspartate and schizophrenia, whether prospective or reverse MR. However, it is important to note that these associations necessitate additional research for further validation.

On the regulation of human D-aspartate oxidase

The human flavoenzyme D-aspartate oxidase (hDASPO) controls the level of D-aspartate in the brain, a molecule acting as an agonist of NMDA receptors and modulator of AMPA and mGlu5 receptors. hDASPO-induced D-aspartate degradation prevents age-dependent deterioration of brain functions and is related to psychiatric disorders such as schizophrenia and autism. Notwithstanding this crucial role, less is known about hDASPO regulation. Here, we report that hDASPO is nitrosylated in vitro, while no evidence of sulfhydration and phosphorylation is apparent: nitrosylation affects the activity of the human flavoenzyme to a limited extent. Furthermore, hDASPO interacts with the primate-specific protein pLG72 (a well-known negative chaperone of D-amino acid oxidase, the enzyme deputed to D-serine degradation in the human brain), yielding a ~114 kDa complex, with a micromolar dissociation constant, promoting the flavoenzyme inactivation. At the cellular level, pLG72 and hDASPO generate a cytosolic complex: the expression of pLG72 negatively affects the hDASPO level by reducing its half-life. We propose that pLG72 binding may represent a protective mechanism aimed at avoiding cytotoxicity due to H2 O2 produced by the hDASPO enzymatic degradation of D-aspartate, especially before the final targeting to peroxisomes.

Amino acid metabolism in health and disease

Amino acids are the building blocks of protein synthesis. They are structural elements and energy sources of cells necessary for normal cell growth, differentiation and function. Amino acid metabolism disorders have been linked with a number of pathological conditions, including metabolic diseases, cardiovascular diseases, immune diseases, and cancer. In the case of tumors, alterations in amino acid metabolism can be used not only as clinical indicators of cancer progression but also as therapeutic strategies. Since the growth and development of tumors depend on the intake of foreign amino acids, more and more studies have targeted the metabolism of tumor-related amino acids to selectively kill tumor cells. Furthermore, immune-related studies have confirmed that amino acid metabolism regulates the function of effector T cells and regulatory T cells, affecting the function of immune cells. Therefore, studying amino acid metabolism associated with disease and identifying targets in amino acid metabolic pathways may be helpful for disease treatment. This article mainly focuses on the research of amino acid metabolism in tumor-oriented diseases, and reviews the research and clinical research progress of metabolic diseases, cardiovascular diseases and immune-related diseases related to amino acid metabolism, in order to provide theoretical basis for targeted therapy of amino acid metabolism.

Promising applications of D-amino acids in periprosthetic joint infection

Due to the rise in our aging population, a disproportionate demand for total joint arthroplasty (TJA) in the elderly is forecast. Periprosthetic joint infection (PJI) represents one of the most challenging complications that can occur following TJA, and as the number of primary and revision TJAs continues to rise, an increasing PJI burden is projected. Despite advances in operating room sterility, antiseptic protocols, and surgical techniques, approaches to prevent and treat PJI remain difficult, primarily due to the formation of microbial biofilms. This difficulty motivates researchers to continue searching for an effective antimicrobial strategy. The dextrorotatory-isoforms of amino acids (D-AAs) are essential components of peptidoglycan within the bacterial cell wall, providing strength and structural integrity in a diverse range of species. Among many tasks, D-AAs regulate cell morphology, spore germination, and bacterial survival, evasion, subversion, and adhesion in the host immune system. When administered exogenously, accumulating data have demonstrated that D-AAs play a pivotal role against bacterial adhesion to abiotic surfaces and subsequent biofilm formation; furthermore, D-AAs have substantial efficacy in promoting biofilm disassembly. This presents D-AAs as promising and novel targets for future therapeutic approaches. Despite their emerging antibacterial efficacy, their role in disrupting PJI biofilm formation, the disassembly of established TJA biofilm, and the host bone tissue response remains largely unexplored. This review aims to examine the role of D-AAs in the context of TJAs. Data to date suggest that D-AA bioengineering may serve as a promising future strategy in the prevention and treatment of PJI.

The Role of D-Serine and D-Aspartate in the Pathogenesis and Therapy of Treatment-Resistant Schizophrenia

Schizophrenia (Sch) is a severe and widespread mental disorder. Antipsychotics (APs) of the first and new generations as the first-line treatment of Sch are not effective in about a third of cases and are also unable to treat negative symptoms and cognitive deficits of schizophrenics. This explains the search for new therapeutic strategies for a disease-modifying therapy for treatment-resistant Sch (TRS). Biological compounds are of great interest to researchers and clinicians, among which D-Serine (D-Ser) and D-Aspartate (D-Asp) are among the promising ones. The Sch glutamate theory suggests that neurotransmission dysfunction caused by glutamate N-methyl-D-aspartate receptors (NMDARs) may represent a primary deficiency in this mental disorder and play an important role in the development of TRS. D-Ser and D-Asp are direct NMDAR agonists and may be involved in modulating the functional activity of dopaminergic neurons. This narrative review demonstrates both the biological role of D-Ser and D-Asp in the normal functioning of the central nervous system (CNS) and in the pathogenesis of Sch and TRS. Particular attention is paid to D-Ser and D-Asp as promising components of a nutritive disease-modifying therapy for TRS.

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.

Emerging imaging methods to study whole-brain function in rodent models

In the past decade, the idea that single populations of neurons support cognition and behavior has gradually given way to the realization that connectivity matters and that complex behavior results from interactions between remote yet anatomically connected areas that form specialized networks. In parallel, innovation in brain imaging techniques has led to the availability of a broad set of imaging tools to characterize the functional organization of complex networks. However, each of these tools poses significant technical challenges and faces limitations, which require careful consideration of their underlying anatomical, physiological, and physical specificity. In this review, we focus on emerging methods for measuring spontaneous or evoked activity in the brain. We discuss methods that can measure large-scale brain activity (directly or indirectly) with a relatively high temporal resolution, from milliseconds to seconds. We further focus on methods designed for studying the mammalian brain in preclinical models, specifically in mice and rats. This field has seen a great deal of innovation in recent years, facilitated by concomitant innovation in gene-editing techniques and the possibility of more invasive recordings. This review aims to give an overview of currently available preclinical imaging methods and an outlook on future developments. This information is suitable for educational purposes and for assisting scientists in choosing the appropriate method for their own research question.

The M1/M4 preferring muscarinic agonist xanomeline modulates functional connectivity and NMDAR antagonist-induced changes in the mouse brain

Cholinergic drugs acting at M1/M4 muscarinic receptors hold promise for the treatment of symptoms associated with brain disorders characterized by cognitive impairment, mood disturbances, or psychosis, such as Alzheimer's disease or schizophrenia. However, the brain-wide functional substrates engaged by muscarinic agonists remain poorly understood. Here we used a combination of pharmacological fMRI (phMRI), resting-state fMRI (rsfMRI), and resting-state quantitative EEG (qEEG) to investigate the effects of a behaviorally active dose of the M1/M4-preferring muscarinic agonist xanomeline on brain functional activity in the rodent brain. We investigated both the effects of xanomeline per se and its modulatory effects on signals elicited by the NMDA-receptor antagonists phencyclidine (PCP) and ketamine. We found that xanomeline induces robust and widespread BOLD signal phMRI amplitude increases and decreased high-frequency qEEG spectral activity. rsfMRI mapping in the mouse revealed that xanomeline robustly decreased neocortical and striatal connectivity but induces focal increases in functional connectivity within the nucleus accumbens and basal forebrain. Notably, xanomeline pre-administration robustly attenuated both the cortico-limbic phMRI response and the fronto-hippocampal hyper-connectivity induced by PCP, enhanced PCP-modulated functional connectivity locally within the nucleus accumbens and basal forebrain, and reversed the gamma and high-frequency qEEG power increases induced by ketamine. Collectively, these results show that xanomeline robustly induces both cholinergic-like neocortical activation and desynchronization of functional networks in the mammalian brain. These effects could serve as a translatable biomarker for future clinical investigations of muscarinic agents, and bear mechanistic relevance for the putative therapeutic effect of these class of compounds in brain 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.

Development of type I/II oligodendrocytes regulated by teneurin-4 in the murine spinal cord

In the spinal cord, the axonal tracts with various caliber sizes are myelinated by oligodendrocytes and function as high-velocity ways for motor and sensory nerve signals. In some neurological disorders, such as multiple sclerosis, demyelination of small caliber axons is observed in the spinal cord. While type I/II oligodendrocytes among the four types are known to myelinate small diameter axons, their characteristics including identification of regulating molecules have not been understood yet. Here, we first found that in the wild-type mouse spinal cord, type I/II oligodendrocytes, positive for carbonic anhydrase II (CAII), were located in the corticospinal tract, fasciculus gracilis, and the inside part of ventral funiculus, in which small diameter axons existed. The type I/II oligodendrocytes started to appear between postnatal day (P) 7 and 11. We further analyzed the type I/II oligodendrocytes in the mutant mice, whose small diameter axons were hypomyelinated due to the deficiency of teneurin-4. In the teneurin-4 deficient mice, type I/II oligodendrocytes were significantly reduced, and the onset of the defect was at P11. Our results suggest that CAII-positive type I/II oligodendrocytes myelinate small caliber axons in the spinal cord and teneurin-4 is the responsible molecule for the generation of type I/II oligodendrocytes.

Glycine Signaling in the Framework of Dopamine-Glutamate Interaction and Postsynaptic Density. Implications for Treatment-Resistant Schizophrenia

Treatment-resistant schizophrenia (TRS) or suboptimal response to antipsychotics affects almost 30% of schizophrenia (SCZ) patients, and it is a relevant clinical issue with significant impact on the functional outcome and on the global burden of disease. Among putative novel treatments, glycine-centered therapeutics (i.e. sarcosine, glycine itself, D-Serine, and bitopertin) have been proposed, based on a strong preclinical rationale with, however, mixed clinical results. Therefore, a better appraisal of glycine interaction with the other major players of SCZ pathophysiology and specifically in the framework of dopamine - glutamate interactions is warranted. New methodological approaches at cutting edge of technology and drug discovery have been applied to study the role of glycine in glutamate signaling, both at presynaptic and post-synaptic level and have been instrumental for unveiling the role of glycine in dopamine-glutamate interaction. Glycine is a non-essential amino acid that plays a critical role in both inhibitory and excitatory neurotransmission. In caudal areas of central nervous system (CNS), such as spinal cord and brainstem, glycine acts as a powerful inhibitory neurotransmitter through binding to its receptor, i.e. the Glycine Receptor (GlyR). However, glycine also works as a co-agonist of the N-Methyl-D-Aspartate receptor (NMDAR) in excitatory glutamatergic neurotransmission. Glycine concentration in the synaptic cleft is finely tuned by glycine transporters, i.e. GlyT1 and GlyT2, that regulate the neurotransmitter's reuptake, with the first considered a highly potential target for psychosis therapy. Reciprocal regulation of dopamine and glycine in forebrain, glycine modulation of glutamate, glycine signaling interaction with postsynaptic density proteins at glutamatergic synapse, and human genetics of glycinergic pathways in SCZ are tackled in order to highlight the exploitation of this neurotransmitters and related molecules in SCZ and TRS.

Common functional networks in the mouse brain revealed by multi-centre resting-state fMRI analysis

Preclinical applications of resting-state functional magnetic resonance imaging (rsfMRI) offer the possibility to non-invasively probe whole-brain network dynamics and to investigate the determinants of altered network signatures observed in human studies. Mouse rsfMRI has been increasingly adopted by numerous laboratories worldwide. Here we describe a multi-centre comparison of 17 mouse rsfMRI datasets via a common image processing and analysis pipeline. Despite prominent cross-laboratory differences in equipment and imaging procedures, we report the reproducible identification of several large-scale resting-state networks (RSN), including a mouse default-mode network, in the majority of datasets. A combination of factors was associated with enhanced reproducibility in functional connectivity parameter estimation, including animal handling procedures and equipment performance. RSN spatial specificity was enhanced in datasets acquired at higher field strength, with cryoprobes, in ventilated animals, and under medetomidine-isoflurane combination sedation. Our work describes a set of representative RSNs in the mouse brain and highlights key experimental parameters that can critically guide the design and analysis of future rodent rsfMRI investigations.

D-Aspartate treatment attenuates myelin damage and stimulates myelin repair

Glutamate signaling may orchestrate oligodendrocyte precursor cell (OPC) development and myelin regeneration through the activation of glutamate receptors at OPC‐neuron synapses. D‐Aspartate is a D‐amino acid exerting modulatory actions at glutamatergic synapses. Chronic administration of D‐Aspartate has been proposed as therapeutic treatment in diseases related to myelin dysfunction and NMDA receptors hypofunction, including schizophrenia and cognitive deficits. Here, we show, by using an in vivo remyelination model, that administration of D‐Aspartate during remyelination improved motor coordination, accelerated myelin recovery, and significantly increased the number of small‐diameter myelinated axons. Chronically administered during demyelination, D‐Aspartate also attenuated myelin loss and inflammation. Interestingly, D‐Aspartate exposure stimulated OPC maturation and accelerated developmental myelination in organotypic cerebellar slices. D‐Aspartate promoting effects on OPC maturation involved the activation of glutamate transporters, AMPA and NMDA receptors, and the Na⁺/Ca²⁺ exchanger NCX3. While blocking NMDA or NCX3 significantly prevented D‐Aspartate‐induced [Ca²⁺]_i oscillations, blocking AMPA and glutamate transporters prevented both the initial and oscillatory [Ca²⁺]_i response as well as D‐Aspartate‐induced inward currents in OPC. Our findings reveal that D‐Aspartate treatment may represent a novel strategy for promoting myelin recovery.

Structural determinants for substrate specificity of flavoenzymes oxidizing d-amino acids

The oxidation of d-amino acids is relevant to neurodegenerative diseases, detoxification, and nutrition in microorganisms and mammals. It is also important for the resolution of racemic amino acid mixtures and the preparation of chiral building blocks for the pharmaceutical and food industry. Considerable biochemical and structural knowledge has been accrued in recent years on the enzymes that carry out the oxidation of the C_α-N bond of d-amino acids. These enzymes contain FAD as a required coenzyme, share similar overall three-dimensional folds and highly conserved active sites, but differ in their specificity for substrates with neutral, anionic, or cationic side-chains. Here, we summarize the current biochemical and structural knowledge regarding substrate specificity on d-amino acid oxidase, d-aspartate oxidase, and d-arginine dehydrogenase for which a wealth of biochemical and structural studies is available.

DNA methylation landscape of the genes regulating D-serine and D-aspartate metabolism in post-mortem brain from controls and subjects with schizophrenia

The spatio-temporal regulation of genes involved in the synthesis and degradation of D-serine and D-aspartate such as serine racemase (SR), D-amino acid oxidase (DAO), G72 and D-aspartate oxidase (DDO), play pivotal roles in determining the correct levels of these D-amino acids in the human brain. Here we provide a comprehensive analysis of mRNA expression and DNA methylation status of these genes in post-mortem samples from hippocampus, dorsolateral prefrontal cortex, and cerebellum from patients with schizophrenia and non-psychiatric controls. DNA methylation analysis was performed at an ultradeep level, measuring individual epialleles frequency by single molecule approach. Differential CpG methylation and expression was detected across different brain regions, although no significant correlations were found with diagnosis. G72 showed the highest CpG and non-CpG methylation degree, which may explain the repression of G72 transcription in the brain regions considered here. Conversely, in line with the sustained SR mRNA expression in the analyzed areas, very low methylation levels were detected at this gene’s regulatory regions. Furthermore, for DAO and DDO, our single-molecule methylation approach demonstrated that analysis of epiallele distribution was able to detect differences in DNA methylation representing area-specific methylation signatures, which are likely not detectable with targeted or genome-wide classic methylation analyses.

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.

Rat d-aspartate oxidase is more similar to the human enzyme than the mouse enzyme

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, as well as chronic pain. Thus, appropriate regulation of d-aspartate is believed to be important for maintaining proper neural activity in the nervous system. Accordingly, much attention has been paid to the role(s) of DDO in the metabolism of d-aspartate in vivo, and the physiological functions of DDO have been actively investigated using experimental rats and mice. However, detailed characterisation of rat DDO has not yet been performed, and little is known about species-specific differences in the properties of mammalian DDOs. In this study, the structural and enzymatic properties of purified recombinant rat, mouse and human DDOs were examined and compared. The results showed that rat DDO is more similar to human DDO than to mouse DDO. This work provides useful insight into the use of rats as an experimental model for investigating the biological significance of human DDO and/or d-aspartate. This article is part of a Special Issue entitled: d-Amino acids: biology in the mirror, edited by Dr. Loredano Pollegioni, Dr. Jean-Pierre Mothet and Dr. Molla Gianluca.

Identifying Rodent Resting-State Brain Networks with Independent Component Analysis

Rodent models have opened the door to a better understanding of the neurobiology of brain disorders and increased our ability to evaluate novel treatments. Resting-state functional magnetic resonance imaging (rs-fMRI) allows for in vivo exploration of large-scale brain networks with high spatial resolution. Its application in rodents affords researchers a powerful translational tool to directly assess/explore the effects of various pharmacological, lesion, and/or disease states on known neural circuits within highly controlled settings. Integration of animal and human research at the molecular-, systems-, and behavioral-levels using diverse neuroimaging techniques empowers more robust interrogations of abnormal/ pathological processes, critical for evolving our understanding of neuroscience. We present a comprehensive protocol to evaluate resting-state brain networks using Independent Component Analysis (ICA) in rodent model. Specifically, we begin with a brief review of the physiological basis for rs-fMRI technique and overview of rs-fMRI studies in rodents to date, following which we provide a robust step-by-step approach for rs-fMRI investigation including data collection, computational preprocessing, and brain network analysis. Pipelines are interwoven with underlying theory behind each step and summarized methodological considerations, such as alternative methods available and current consensus in the literature for optimal results. The presented protocol is designed in such a way that investigators without previous knowledge in the field can implement the analysis and obtain viable results that reliably detect significant differences in functional connectivity between experimental groups. Our goal is to empower researchers to implement rs-fMRI in their respective fields by incorporating technical considerations to date into a workable methodological framework.

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.

Intranasal Oxytocin and Vasopressin Modulate Divergent Brainwide Functional Substrates

The neuropeptides oxytocin (OXT) and vasopressin (AVP) have been identified as modulators of emotional social behaviors and associated with neuropsychiatric disorders characterized by social dysfunction. Experimental and therapeutic use of OXT and AVP via the intranasal route is the subject of extensive clinical research. However, the large-scale functional substrates directly engaged by these peptides and their functional dynamics remain elusive. By using cerebral blood volume (CBV) weighted fMRI in the mouse, we show that intranasal administration of OXT rapidly elicits the transient activation of cortical regions and a sustained activation of hippocampal and forebrain areas characterized by high oxytocin receptor density. By contrast, intranasal administration of AVP produced a robust and sustained deactivation in cortico-parietal, thalamic and mesolimbic regions. Importantly, intravenous administration of OXT and AVP did not recapitulate the patterns of modulation produced by intranasal dosing, supporting a central origin of the observed functional changes. In keeping with this notion, hippocampal local field potential recordings revealed multi-band power increases upon intranasal OXT administration. We also show that the selective OXT-derivative TGOT reproduced the pattern of activation elicited by OXT and that the deletion of OXT receptors does not affect AVP-mediated deactivation. Collectively, our data document divergent modulation of brainwide neural systems by intranasal administration of OXT and AVP, an effect that involves key substrates of social and emotional behavior. The observed divergence calls for a deeper investigation of the systems-level mechanisms by which exogenous OXT and AVP modulate brain function and exert their putative therapeutic effects.

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.

Decreased free d-aspartate levels are linked to enhanced d-aspartate oxidase activity in the dorsolateral prefrontal cortex of schizophrenia patients

It is long acknowledged that the N-methyl d-aspartate receptor co-agonist, d-serine, plays a crucial role in several N-methyl d-aspartate receptor-mediated physiological and pathological processes, including schizophrenia. Besides d-serine, another free d-amino acid, d-aspartate, is involved in the activation of N-methyl d-aspartate receptors acting as an agonist of this receptor subclass, and is abundantly detected in the developing human brain. Based on the hypothesis of N-methyl d-aspartate receptor hypofunction in the pathophysiology of schizophrenia and considering the ability of d-aspartate and d-serine to stimulate N-methyl d-aspartate receptor-dependent transmission, in the present work we assessed the concentration of these two d-amino acids in the post-mortem dorsolateral prefrontal cortex and hippocampus of patients with schizophrenia and healthy subjects. Moreover, in this cohort of post-mortem brain samples we investigated the spatiotemporal variations of d-aspartate and d-serine. Consistent with previous work, we found that d-aspartate content was selectively decreased by around 30% in the dorsolateral prefrontal cortex, but not in the hippocampus, of schizophrenia-affected patients, compared to healthy subjects. Interestingly, such selective reduction was associated to greater (around 25%) cortical activity of the enzyme responsible for d-aspartate catabolism, d-aspartate oxidase. Conversely, no significant changes were found in the methylation state and transcription of DDO gene in patients with schizophrenia, compared to control individuals, as well as in the expression levels of serine racemase, the major enzyme responsible for d-serine biosynthesis, which also catalyzes aspartate racemization. These results reveal the potential involvement of altered d-aspartate metabolism in the dorsolateral prefrontal cortex as a factor contributing to dysfunctional N-methyl d-aspartate receptor-mediated transmission in schizophrenia.

Altered metabolism of an amino acid activator of ion channels in the brain could explain dysfunctional nerve signaling in schizophrenia. Researchers in Italy led by Alessandro Usiello from Ceinge Biotecnologie Avanzate and Loredano Pollegioni from the University of Insubria measured the levels of two amino acids—D-aspartate and D-serine—in post-mortem tissues taken from two brain regions of patients with and without schizophrenia. Both amino acids activate the N-methyl D-aspartate receptor, which is known to be less active in people with schizophrenia. The researchers found a mild increase in D-serine levels but a major decrease in D-aspartate in the schizophrenia patients’ dorsolateral prefrontal cortex (DLPFC), a memory and reasoning part of the brain, but not in the hippocampus. They also documented a greater activity of the enzyme responsible for D-aspartate breakdown in the DLPFC.

Tracking the evolution of epialleles during neural differentiation and brain development: D-Aspartate oxidase as a model gene

We performed ultra-deep methylation analysis at single molecule level of the promoter region of developmentally regulated D-Aspartate oxidase (Ddo), as a model gene, during brain development and embryonic stem cell neural differentiation. Single molecule methylation analysis enabled us to establish the effective epiallele composition within mixed or pure brain cell populations. In this framework, an epiallele is defined as a specific combination of methylated CpG within Ddo locus and can represent the epigenetic haplotype revealing a cell-to-cell methylation heterogeneity. Using this approach, we found a high degree of polymorphism of methylated alleles (epipolymorphism) evolving in a remarkably conserved fashion during brain development. The different sets of epialleles mark stage, brain areas, and cell type and unravel the possible role of specific CpGs in favoring or inhibiting local methylation. Undifferentiated embryonic stem cells showed non-organized distribution of epialleles that apparently originated by stochastic methylation events on individual CpGs. Upon neural differentiation, despite detecting no changes in average methylation, we observed that the epiallele distribution was profoundly different, gradually shifting toward organized patterns specific to the glial or neuronal cell types. Our findings provide a deep view of gene methylation heterogeneity in brain cell populations promising to furnish innovative ways to unravel mechanisms underlying methylation patterns generation and alteration in brain diseases.

The frontal cortex as a network hub controlling mood and cognition: Probing its neurochemical substrates for improved therapy of psychiatric and neurological disorders

The highly-interconnected and neurochemically-rich frontal cortex plays a crucial role in the regulation of mood and cognition, domains disrupted in depression and other central nervous system disorders, and it is an important site of action for their therapeutic control. For improving our understanding of the function and dysfunction of the frontal cortex, and for identifying improved treatments, quantification of extracellular pools of neuromodulators by microdialysis in freely-moving rodents has proven indispensable. This approach has revealed a complex mesh of autoreceptor and heteroceptor interactions amongst monoaminergic pathways, and led from selective 5-HT reuptake inhibitors to novel classes of multi-target drugs for treating depression like the mixed α₂-adrenoceptor/5-HT reuptake inhibitor, S35966, and the clinically-launched vortioxetine and vilazodone. Moreover, integration of non-monoaminergic actions resulted in the discovery and development of the innovative melatonin receptor agonist/5-HT_2C receptor antagonist, Agomelatine. Melatonin levels, like those of corticosterone and the "social hormone", oxytocin, can now be quantified by microdialysis over the full 24 h daily cycle. Further, the introduction of procedures for measuring extracellular histamine and acetylcholine has provided insights into strategies for improving cognition by, for example, blockade of 5-HT₆ and/or dopamine D₃ receptors. The challenge of concurrently determining extracellular levels of GABA, glutamate, d-serine, glycine, kynurenate and other amino acids, and of clarifying their interactions with monoamines, has also been resolved. This has proven important for characterizing the actions of glycine reuptake inhibitors that indirectly augment transmission at N-methyl-d-aspartate receptors, and of "glutamatergic antidepressants" like ketamine, mGluR5 antagonists and positive modulators of AMPA receptors (including S47445). Most recently, quantification of the neurotoxic proteins Aβ42 and Tau has extended microdialysis studies to the pathogenesis of neurodegenerative disorders, and another frontier currently being broached is microRNAs. The present article discusses the above themes, focusses on recent advances, highlights opportunities for clinical "translation", and suggests avenues for further progress.

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.

Large-scale functional connectivity networks in the rodent brain

Resting-state functional Magnetic Resonance Imaging (rsfMRI) of the human brain has revealed multiple large-scale neural networks within a hierarchical and complex structure of coordinated functional activity. These distributed neuroanatomical systems provide a sensitive window on brain function and its disruption in a variety of neuropathological conditions. The study of macroscale intrinsic connectivity networks in preclinical species, where genetic and environmental conditions can be controlled and manipulated with high specificity, offers the opportunity to elucidate the biological determinants of these alterations. While rsfMRI methods are now widely used in human connectivity research, these approaches have only relatively recently been back-translated into laboratory animals. Here we review recent progress in the study of functional connectivity in rodent species, emphasising the ability of this approach to resolve large-scale brain networks that recapitulate neuroanatomical features of known functional systems in the human brain. These include, but are not limited to, a distributed set of regions identified in rats and mice that may represent a putative evolutionary precursor of the human default mode network (DMN). The impact and control of potential experimental and methodological confounds are also critically discussed. Finally, we highlight the enormous potential and some initial application of connectivity mapping in transgenic models as a tool to investigate the neuropathological underpinnings of the large-scale connectional alterations associated with human neuropsychiatric and neurological conditions. We conclude by discussing the translational potential of these methods in basic and applied neuroscience.

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.

Identification of Novel D-Aspartate Oxidase Inhibitors by in Silico Screening and Their Functional and Structural Characterization in Vitro

D-Aspartate oxidase (DDO) is a degradative enzyme that is stereospecific for acidic D-amino acids, including D-aspartate, a potential agonist of the N-methyl-D-aspartate (NMDA) receptor. Dysfunction of NMDA receptor-mediated neurotransmission has been implicated in the onset of various mental disorders, such as schizophrenia. Hence, a DDO inhibitor that increases the brain levels of D-aspartate and thereby activates NMDA receptor function is expected to be a useful compound. To search for potent DDO inhibitor(s), a large number of compounds were screened in silico, and several compounds were identified as candidates. They were then characterized and evaluated as novel DDO inhibitors in vitro (e.g., the inhibitor constant value of 5-aminonicotinic acid for human DDO was 3.80 μM). The present results indicate that some of these compounds may serve as lead compounds for the development of a clinically useful DDO inhibitor and as active site probes to elucidate the structure-function relationships of DDO.