G72 primate-specific gene: a still enigmatic element in psychiatric disorders

Sex-specific cortical, hippocampal and thalamic whole genome transcriptome data from controls and a G72 schizophrenia mouse model

OBJECTIVES

The G72 mouse model of schizophrenia represents a well-known model that was generated to meet the main translational criteria of isomorphism, homology and predictability of schizophrenia to a maximum extent. In order to get a more detailed view of the complex etiopathogenesis of schizophrenia, whole genome transcriptome studies turn out to be indispensable. Here we carried out microarray data collection based on RNA extracted from the retrosplenial cortex, hippocampus and thalamus of G72 transgenic and wild-type control mice. Experimental animals were age-matched and importantly, both sexes were considered separately.

DATA DESCRIPTION

The isolated RNA from all three brain regions was purified, quantified und quality controlled before initiation of the hybridization procedure with SurePrint G3 Mouse Gene Expression v2 8  ×  60 K microarrays. Following immunofluorescent measurement und preprocessing of image data, raw transcriptome data from G72 mice and control animals were extracted and uploaded in a public database. Our data allow insight into significant alterations in gene transcript levels in G72 mice and enable the reader/user to perform further complex analyses to identify potential age-, sex- and brain-region-specific alterations in transcription profiles and related pathways. The latter could facilitate biomarker identification and drug research and development in schizophrenia research.

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.

Diagnosing Alzheimer's Disease Specifically and Sensitively With pLG72 and Cystine/Glutamate Antiporter SLC7A11 AS Blood Biomarkers

BACKGROUND

Reliable blood biomarkers for Alzheimer's disease (AD) have been lacking. The D-amino acids oxidase modulator (named pLG72) modulates glutamate N-methyl-D-aspartate receptor activity. The cystine/glutamate antiporter contains a SLC7A11 subunit, which mediates glutamate release. This study aimed to determine the accuracy of pLG72 protein and SLC7A11 mRNA in diagnosing AD.

METHODS

This study enrolled 130 healthy controls and 109 unmatched AD patients; among them, 40 controls and 70 patients were selected to match by age. We measured their pLG72 protein in plasma and SLC7A11 mRNA in white blood cells.

RESULTS

AD patients had markedly higher pLG72 levels and SLC7A11 mRNA ΔCT values than healthy controls (in both unmatched and matched cohorts; all 4 P values <.001). The receiver operating characteristics analysis in the unmatched cohorts demonstrated that the pLG72 level had a high specificity (0.900) at the optimal cutoff value of 2.3285, the ΔCT of SLC7A11 mRNA displayed an excellent sensitivity (0.954) at the cutoff of 12.185, and the combined value of pLG72 and SLC7A11 ΔCT determined a favorable area under the curve (AUC) (0.882) at the cutoff of 21.721. The AUC of the combined value surpassed that of either biomarker. The specificity, sensitivity, and AUC of the matched cohort were like those of the unmatched cohort.

CONCLUSIONS

The findings suggest that pLG72 protein and SLC7A11 mRNA can distinguish AD patients from healthy controls with excellent specificity and sensitivity, respectively. The combination of pLG72 and SLC7A11 yields better AUC than either, suggesting the superiority of simultaneously measuring both biomarkers in identifying AD patients.

Biochemical Properties and Physiological Functions of pLG72: Twenty Years of Investigations

In 2002, the novel human gene G72 was associated with schizophrenia susceptibility. This gene encodes a small protein of 153 amino acids, named pLG72, which represents a rare case of primate-specific protein. In particular, the rs2391191 single nucleotide polymorphism (resulting in in the R30K substitution) was robustly associated to schizophrenia and bipolar disorder. In this review, we aim to summarize the results of 20 years of biochemical investigations on pLG72. The main known role of pLG72 is related to its ability to bind and inactivate the flavoenzyme d-amino acid oxidase, i.e., the enzyme that controls the catabolism of d-serine, the main NMDA receptor coagonist in the brain. pLG72 was proposed to target the cytosolic form of d-amino acid oxidase for degradation, preserving d-serine and protecting the cell from oxidative stress generated by hydrogen peroxide produced by the flavoenzyme reaction. Anyway, pLG72 seems to play additional roles, such as affecting mitochondrial functions. The level of pLG72 in the human body is still a controversial issue because of its low expression and challenging detection. Anyway, the intriguing hypothesis that pLG72 level in blood could represent a suitable marker of Alzheimer’s disease progression (a suggestion not sufficiently established yet) merits further investigations.

Multi-Omics Analysis Reveals Myelin, Presynaptic and Nicotinate Alterations in the Hippocampus of G72/G30 Transgenic Mice

The primate-specific G72/G30 gene locus has been associated with major psychiatric disorders, such as schizophrenia and bipolar disorder. We have previously generated transgenic mice which carry the G72/G30 locus and express the longest G72 splice variant (LG72) protein encoded by this locus with schizophrenia-related symptoms. Here, we used a multi-omics approach, including quantitative proteomics and metabolomics to investigate molecular alterations in the hippocampus of G72/G30 transgenic (G72Tg) mice. Our proteomics analysis revealed decreased expression of myelin-related proteins and NAD-dependent protein deacetylase sirtuin-2 (Sirt2) as well as increased expression of the scaffolding presynaptic proteins bassoon (Bsn) and piccolo (Pclo) and the cytoskeletal protein plectin (Plec1) in G72Tg compared to wild-type (WT) mice. Metabolomics analysis indicated decreased levels of nicotinate in G72Tg compared to WT hippocampi. Decreased hippocampal protein expression for selected proteins, namely myelin oligodentrocyte glycoprotein (Mog), Cldn11 and myelin proteolipid protein (Plp), was confirmed with Western blot in a larger population of G72Tg and WT mice. The identified molecular pathway alterations shed light on the hippocampal function of LG72 protein in the context of neuropsychiatric phenotypes.

D-Amino Acids as a Biomarker in Schizophrenia

D-amino acids may play key roles for specific physiological functions in different organs including the brain. Importantly, D-amino acids have been detected in several neurological disorders such as schizophrenia, amyotrophic lateral sclerosis, and age-related disorders, reflecting the disease conditions. Relationships between D-amino acids and neurophysiology may involve the significant contribution of D-Serine or D-Aspartate to the synaptic function, including neurotransmission and synaptic plasticity. Gut-microbiota could play important roles in the brain-function, since bacteria in the gut provide a significant contribution to the host pool of D-amino acids. In addition, the alteration of the composition of the gut microbiota might lead to schizophrenia. Furthermore, D-amino acids are known as a physiologically active substance, constituting useful biomarkers of several brain disorders including schizophrenia. In this review, we wish to provide an outline of the roles of D-amino acids in brain health and neuropsychiatric disorders with a focus on schizophrenia, which may shed light on some of the superior diagnoses and/or treatments of schizophrenia.

The Role of D-Amino Acids in Alzheimer's Disease

Alzheimer's disease (AD), the main cause of dementia worldwide, is characterized by a complex and multifactorial etiology. In large part, excitatory neurotransmission in the central nervous system is mediated by glutamate and its receptors are involved in synaptic plasticity. The N-methyl-D-aspartate (NMDA) receptors, which require the agonist glutamate and a coagonist such as glycine or the D-enantiomer of serine for activation, play a main role here. A second D-amino acid, D-aspartate, acts as agonist of NMDA receptors. D-amino acids, present in low amounts in nature and long considered to be of bacterial origin, have distinctive functions in mammals. In recent years, alterations in physiological levels of various D-amino acids have been linked to various pathological states, ranging from chronic kidney disease to neurological disorders. Actually, the level of NMDA receptor signaling must be balanced to promote neuronal survival and prevent neurodegeneration: this signaling in AD is affected mainly by glutamate availability and modulation of the receptor's functions. Here, we report the experimental findings linking D-serine and D-aspartate, through NMDA receptor modulation, to AD and cognitive functions. Interestingly, AD progression has been also associated with the enzymes related to D-amino acid metabolism as well as with glucose and serine metabolism. Furthermore, the D-serine and D-/total serine ratio in serum have been recently proposed as biomarkers of AD progression. A greater understanding of the role of D-amino acids in excitotoxicity related to the pathogenesis of AD will facilitate novel therapeutic treatments to cure the disease and improve life expectancy.

Yin and Yang in Post-Translational Modifications of Human D-Amino Acid Oxidase

In the central nervous system, the flavoprotein D-amino acid oxidase is responsible for catabolizing D-serine, the main endogenous coagonist of N-methyl-D-aspartate receptor. Dysregulation of D-serine brain levels in humans has been associated with neurodegenerative and psychiatric disorders. This D-amino acid is synthesized by the enzyme serine racemase, starting from the corresponding L-enantiomer, and degraded by both serine racemase (via an elimination reaction) and the flavoenzyme D-amino acid oxidase. To shed light on the role of human D-amino acid oxidase (hDAAO) in D-serine metabolism, the structural/functional relationships of this enzyme have been investigated in depth and several strategies aimed at controlling the enzymatic activity have been identified. Here, we focused on the effect of post-translational modifications: by using a combination of structural analyses, biochemical methods, and cellular studies, we investigated whether hDAAO is subjected to nitrosylation, sulfhydration, and phosphorylation. hDAAO is S-nitrosylated and this negatively affects its activity. In contrast, the hydrogen sulfide donor NaHS seems to alter the enzyme conformation, stabilizing a species with higher affinity for the flavin adenine dinucleotide cofactor and thus positively affecting enzymatic activity. Moreover, hDAAO is phosphorylated in cerebellum; however, the protein kinase involved is still unknown. Taken together, these findings indicate that D-serine levels can be also modulated by post-translational modifications of hDAAO as also known for the D-serine synthetic enzyme serine racemase.

The role of trauma experiences, personality traits, and genotype in maintaining posttraumatic stress disorder symptoms among child survivors of the Wenchuan earthquake

BACKGROUND

Posttraumatic stress disorder (PTSD) is the most prevalent type of psychiatric disorder among children after an earthquake. This study investigated the role of trauma experiences, personality traits, and genotype in the maintenance of PTSD symptoms.

METHODS

In a previous large-scale epidemiological investigation 1 year after the Wenchuan earthquake, 215 children with PTSD symptoms were selected at random with their blood samples collected. All of them were followed up, and their PTSD symptoms were assessed 3 years later. The adolescent version of the UCLA PTSD Reaction Index, the earthquake exposure scale, and the Junior Eysenck Personality Questionnaire were used to determine PTSD symptoms, trauma experiences, and personality traits, respectively. We sequenced candidate genes involved in the regulation of long-term potentiation via NMDA-type receptors to identify the related SNP variations.

RESULTS

Being trapped for a longer period of time, feeling one's own or a family member's life to be in danger, losing a close family member or friend, extraversion, neuroticism, TrkB, G72 and CNTF were found to be associated with the maintenance of PTSD symptoms.

CONCLUSIONS

Experiences, personality traits, and genotype influenced the maintenance of PTSD in child survivors who were considered to be followed up without medicine. This result could help to identify potential targets for treatment and promote the rational allocation of medical resources.

Effect of DAOA genetic variation on white matter alteration in corpus callosum in patients with first-episode schizophrenia

D-amino acid oxidase activator (DAOA) gene, which plays a crucial role in the process of glutamatergic transmission and mitochondrial function, is frequently linked with the liability for schizophrenia. We aimed to investigate whether the variation of DAOA rs2391191 is associated with alterations in white matter integrity of first-episode schizophrenia (FES) patients; and whether it influences the association between white matter integrity, cognitive function and clinical symptoms of schizophrenia. Forty-six patients with FES and forty-nine healthy controls underwent DTI and were genotyped for DAOA rs2391191. Psychopathological assessments were performed by Brief Psychiatric Rating Scale (BPRS) and Scale for Assessment of Negative Symptoms (SANS). Cognitive function was assessed by MATRICS Consensus Cognitive Battery (MCCB). Schizophrenia patients presented lower fractional anisotropy (FA) and higher radial diffusivity (RD), mainly spreading over the corpus callosum and corona radiata compared with healthy controls. Compared with patients carrying G allele, patients with AA showed lower FA in the body of corpus callosum, and higher RD in the genu of corpus callosum, right superior and anterior corona radiata, and left posterior corona radiata. In patients carrying G allele, FA in body of corpus callosum was positively correlated with working memory, RD in genu of corpus callosum was negatively associated with the speed of processing, working memory, and the composite score of MCCB, while no significant correlations were found in AA homozygotes. In our study, patients with FES presented abnormal white matter integrity in corpus callosum and corona radiata. Furthermore, this abnormality was associated with the genetic variation of DAOA rs2391191, with AA homozygotes showing less white matter integrity in the corpus callosum. Our findings possibly provide further support to the evidence that DAOA regulates the process of glutamatergic neurotransmission and mitochondrial function in the pathophysiological mechanism of schizophrenia.

Is the primate-specific protein pLG72 affecting SOD1 functionality and superoxide formation?

pLG72 is a primate-specific protein of enigmatic function that was proposed to modulate mitochondria fragmentation and the activity of the peroxisomal enzyme D-amino acid oxidase (DAAO). DAAO is deputed to degradation of the NMDA receptor co-agonist D-serine in human brain and the R199W substitution in DAAO was identified in a familial case of amyotrophic lateral sclerosis (ALS). A recent work reported that U87 glioblastoma cells ectopically expressing pLG72 showed a lower proliferation, produced superoxide radicals, induced SOD1 aggregation and decreased its activity. Because of the role of SOD1 in eliminating ROS species and its relevance in ALS we evaluated the link between pLG72 and SOD1 using both wild-type pLG72 and its R30K variant related to schizophrenia susceptibility. In vitro studies on recombinant proteins excluded the establishment of a stable complex and that pLG72 could affect SOD1 activity and stability. At cellular level, ectopic expression of pLG72 in glioblastoma U87 cells did not affect cell viability and ROS/superoxide production: only caspase activity (a marker of apoptosis) was slightly increased in cells expressing the R30K pLG72 variant. SOD1 and pLG72 did not colocalize in transfected U87 glioblastoma cells: pLG72 largely localised to mitochondria and SOD1 was largely cytosolic. Moreover, the ectopic expression of pLG72 appeared not to alter the expression of SOD1 and its aggregation. Altogether, the combination of biochemical and cellular studies allow to exclude that pLG72 modulates SOD1 function and aggregation, thus that it could play a role in ALS susceptibility.

pLG72 levels increase in early phase of Alzheimer's disease but decrease in late phase

pLG72, named as D-amino acid oxidase activator (although it is not an activator of D-amino acid oxidase demonstrated by later studies), in mitochondria has been regarded as an important modulator of D-amino acid oxidase that can regulate the N-methyl-D-aspartate receptor (NMDAR). Both oxidative stress in mitochondria and NMDAR neurotransmission play essential roles in the process of neurodegenerative dementia. The aim of the study was to investigate whether pLG72 levels changed with the severity of neurodegenerative dementia. We enrolled 376 individuals as the overall cohort, consisting of five groups: healthy elderly, amnestic mild cognitive impairment [MCI], mild Alzheimer's disease [AD], moderate AD, and severe AD. pLG72 levels in plasma were measured using Western blotting. The severity of cognitive deficit was principally evaluated by Clinical Dementia Rating Scale. A gender- and age- matched cohort was selected to elucidate the effects of gender and age. pLG72 levels increased in the MCI and mild AD groups when compared to the healthy group. However, pLG72 levels in the moderate and severe AD groups were lower than those in the mild AD group. D-serine level and D- to total serine ratio were significantly different among the five groups. L-serine levels were correlated with the pLG72 levels. The results in the gender- and age- matched cohort were similar to those of the overall cohort. The finding supports the hypothesis of NMDAR hypofunction in early-phase dementia and NMDAR hyperfunction in late-phase dementia. Further studies are warranted to test whether pLG72 could reflect the function of NMDAR.

Human D-Amino Acid Oxidase: Structure, Function, and Regulation

D-Amino acid oxidase (DAAO) is an FAD-containing flavoenzyme that catalyzes with absolute stereoselectivity the oxidative deamination of all natural D-amino acids, the only exception being the acidic ones. This flavoenzyme plays different roles during evolution and in different tissues in humans. Its three-dimensional structure is well conserved during evolution: minute changes are responsible for the functional differences between enzymes from microorganism sources and those from humans. In recent years several investigations focused on human DAAO, mainly because of its role in degrading the neuromodulator D-serine in the central nervous system. D-Serine is the main coagonist of N-methyl D-aspartate receptors, i.e., excitatory amino acid receptors critically involved in main brain functions and pathologic conditions. Human DAAO possesses a weak interaction with the FAD cofactor; thus, in vivo it should be largely present in the inactive, apoprotein form. Binding of active-site ligands and the substrate stabilizes flavin binding, thus pushing the acquisition of catalytic competence. Interestingly, the kinetic efficiency of the enzyme on D-serine is very low. Human DAAO interacts with various proteins, in this way modulating its activity, targeting, and cell stability. The known properties of human DAAO suggest that its activity must be finely tuned to fulfill a main physiological function such as the control of D-serine levels in the brain. At present, studies are focusing on the epigenetic modulation of human DAAO expression and the role of post-translational modifications on its main biochemical properties at the cellular level.

Sodium Benzoate, a D-Amino Acid Oxidase Inhibitor, Added to Clozapine for the Treatment of Schizophrenia: A Randomized, Double-Blind, Placebo-Controlled Trial

BACKGROUND

Clozapine is the last-line antipsychotic agent for refractory schizophrenia. To date, there is no convincing evidence for augmentation on clozapine. Activation of N-methyl-D-aspartate receptors, including inhibition of D-amino acid oxidase that may metabolize D-amino acids, has been reported to be beneficial for patients receiving antipsychotics other than clozapine. This study aimed to examine the efficacy and safety of a D-amino acid oxidase inhibitor, sodium benzoate, for schizophrenia patients who had poor response to clozapine.

METHODS

We conducted a randomized, double-blind, placebo-controlled trial. Sixty schizophrenia inpatients that had been stabilized with clozapine were allocated into three groups for 6 weeks' add-on treatment of 1 g/day sodium benzoate, 2 g/day sodium benzoate, or placebo. The primary outcome measures were Positive and Negative Syndrome Scale (PANSS) total score, Scale for the Assessment of Negative Symptoms, Quality of Life Scale, and Global Assessment of Functioning. Side effects and cognitive functions were also measured.

RESULTS

Both doses of sodium benzoate produced better improvement than placebo in the Scale for the Assessment of Negative Symptoms. The 2 g/day sodium benzoate also produced better improvement than placebo in PANSS-total score, PANSS-positive score, and Quality of Life Scale. Sodium benzoate was well tolerated without evident side effects. The changes of catalase, an antioxidant, were different among the three groups and correlated with the improvement of PANSS-total score and PANSS-positive score in the sodium benzoate group.

CONCLUSIONS

Sodium benzoate adjuvant therapy improved symptomatology of patients with clozapine-resistant schizophrenia. Further studies are warranted to elucidate the optimal dose and treatment duration as well as the mechanisms of sodium benzoate for clozapine-resistant schizophrenia.

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.

Biochemical Properties of Human D-amino Acid Oxidase Variants and Their Potential Significance in Pathologies

The stereoselective flavoenzyme D-amino acid oxidase (DAAO) catalyzes the oxidative deamination of neutral and polar D-amino acids producing the corresponding α-keto acids, ammonia, and hydrogen peroxide. Despite its peculiar and atypical substrates, DAAO is widespread expressed in most eukaryotic organisms. In mammals (and humans in particular), DAAO is involved in relevant physiological processes ranging from D-amino acid detoxification in kidney to neurotransmission in the central nervous system, where DAAO is responsible of the catabolism of D-serine, a key endogenous co-agonist of N-methyl-D-aspartate receptors. Recently, structural and functional studies have brought to the fore the distinctive biochemical properties of human DAAO (hDAAO). It appears to have evolved to allow a strict regulation of its activity, so that the enzyme can finely control the concentration of substrates (such as D-serine in the brain) without yielding to an excessive production of hydrogen peroxide, a potentially toxic reactive oxygen species (ROS). Indeed, dysregulation in D-serine metabolism, likely resulting from altered levels of hDAAO expression and activity, has been implicated in several pathologies, ranging from renal disease to neurological, neurodegenerative, and psychiatric disorders. Only one mutation in DAO gene was unequivocally associated to a human disease. However, several single nucleotide polymorphisms (SNPs) are reported in the database and the biochemical characterization of the corresponding recombinant hDAAO variants is of great interest for investigating the effect of mutations. Here we reviewed recently published data focusing on the modifications of the structural and functional properties induced by amino acid substitutions encoded by confirmed SNPs and on their effect on D-serine cellular levels. The potential significance of the different hDAAO variants in human pathologies will be also discussed.

D-Amino Acid Oxidase-pLG72 Interaction and D-Serine Modulation

pLG72 is a small, primate-specific protein of 153 amino acids. It is the product of the G72 gene, expressed in testis, spinal cord, and brain. The presence of G72 transcript and pLG72 has recurrently been called into question, however G72 mRNA and pLG72 protein levels were higher in blood and brain of patients with schizophrenia than in healthy controls. On the one hand, the SNP rs2391191 corresponding to the R30K substitution in pLG72 was genetically linked to schizophrenia, reduced thickness of the brain cortex in schizophrenia-affected individuals, and altered memory function. Various lines of evidence indicated that pLG72 is a mitochondrial protein, specifically an extrinsic protein bound on the outer membrane. Over the years, pLG72 was proposed to be involved in different functions: (a) overexpression induces mitochondria fragmentation, increasing the numbers of shorter and more mobile ones which could be delivered faster to regions of intense growth and facilitating the dendritic complexity; (b) it might induce oxidative stress by interacting with methionine-R-sulfoxide reductase B2; and (c) it binds and modulates the activity of FMN-containing oxidoreductase of the respiratory complex I. The main role of this protein, however, is related to its binding to the human flavoenzyme D-amino acid oxidase (hDAAO), i.e., the main catabolic enzyme for D-enantiomer of serine. This D-amino acid is a main endogenous coagonist of the N-methyl-D-aspartate type glutamate receptor (NMDAR) involved in main functions such as synaptic plasticity, learning, memory, and excitotoxicity. For this work, we reviewed the recent literature concerning the hDAAO-pLG72 interaction, focusing on the molecular details of the interaction, the effect of hDAAO function and stability, and the cellular effects, especially on D-serine concentration. The main effects related to the pathological R30K substitution are also reported. We have highlighted the gaps in our knowledge of this human protein as well as the relevance of clarifying the molecular details of hDAAO-pLG72 interaction in order to design molecules to modulate hDAAO activity/stability and thus NMDAR function acting at the D-serine cellular level.

Biochemical Properties of Human D-Amino Acid Oxidase

D-amino acid oxidase catalyzes the oxidative deamination of D-amino acids. In the brain, the NMDA receptor coagonist D-serine has been proposed as its physiological substrate. In order to shed light on the mechanisms regulating D-serine concentration at the cellular level, we biochemically characterized human DAAO (hDAAO) in greater depth. In addition to clarify the physical-chemical properties of the enzyme, we demonstrated that divalent ions and nucleotides do not affect flavoenzyme function. Moreover, the definition of hDAAO substrate specificity demonstrated that D-cysteine is the best substrate, which made it possible to propose it as a putative physiological substrate in selected tissues. Indeed, the flavoenzyme shows a preference for hydrophobic amino acids, some of which are molecules relevant in neurotransmission, i.e., D-kynurenine, D-DOPA, and D-tryptophan. hDAAO shows a very low affinity for the flavin cofactor. The apoprotein form exists in solution in equilibrium between two alternative conformations: the one at higher affinity for FAD is favored in the presence of an active site ligand. This may represent a mechanism to finely modulate hDAAO activity by substrate/inhibitor presence. Taken together, the peculiar properties of hDAAO seem to have evolved in order to use this flavoenzyme in different tissues to meet different physiological needs related to D-amino acids.

Controversial Effects of D-Amino Acid Oxidase Activator (DAOA)/G72 on D-Amino Acid Oxidase (DAO) Activity in Human Neuronal, Astrocyte and Kidney Cell Lines: The N-methyl D-aspartate (NMDA) Receptor Hypofunction Point of View

Dysfunction of D-amino acid oxidase (DAO) and DAO activator (DAOA)/G72 genes have been linked to neuropsychiatric disorders. The glutamate hypothesis of schizophrenia has proposed that increased DAO activity leads to decreased D-serine, which subsequently may lead to N-methyl-D-aspartate (NMDA) receptor hypofunction. It has been shown that DAOA binds to DAO and increases its activity. However, there are also studies showing DAOA decreases DAO activity. Thus, the effect of DAOA on DAO is controversial. We aimed to understand the effect of DAOA on DAO activity in neuron-like (SH-SY5Y), astrocyte-like (1321N1) and kidney-like (HEK293) human cell lines. DAO activity was measured based on the release of hydrogen peroxide and its interaction with Amplex Red reagent. We found that DAOA increases DAO activity only in HEK293 cells, but has no effect on DAO activity in SH-SY5Y and 1321N1 cells. This might be because of different signaling pathways, or due to lower DAO and DAOA expression in SH-SY5Y and 1321N1 cells compared to HEK293 cells, but also due to different compartmentalization of the proteins. The lower DAO and DAOA expression in neuron-like SH-SY5Y and astrocyte-like 1321N1 cells might be due to tightly regulated expression, as previously reported in the human post-mortem brain. Our simulation experiments to demonstrate the interaction between DAOA and human DAO (hDAO) showed that hDAO holoenzyme [hDAO with flavine adenine dinucleotide (FAD)] becomes more flexible and misfolded in the presence of DAOA, whereas DAOA had no effect on hDAO apoprotein (hDAO without FAD), which indicate that DAOA inactivates hDAO holoenzyme. Furthermore, patch-clamp analysis demonstrated no effect of DAOA on NMDA receptor activity in NR1/NR2A HEK293 cells. In summary, the interaction between DAO and DAOA seems to be cell type and its biochemical characteristics dependent which still needs to be elucidated.

An emerging role for mitochondrial dynamics in schizophrenia

Abnormal brain development has long been thought to contribute to the pathophysiology of schizophrenia. Impaired dendritic arborization, synaptogenesis, and long term potentiation and memory have been demonstrated in animal models of schizophrenia. In addition to aberrant nervous system development, altered brain metabolism and mitochondrial function has long been observed in schizophrenic patients. Single nucleotide polymorphisms in the mitochondrial genome as well as impaired mitochondrial function have both been associated with increased risk for developing schizophrenia. Mitochondrial function in neurons is highly dependent on fission, fusion, and transport of the organelle, collectively referred to as mitochondrial dynamics. Indeed, there is mounting evidence that mitochondrial dynamics strongly influences neuron development and synaptic transmission. While there are a few studies describing altered mitochondrial shape in schizophrenic patients, as well as in animal and in vitro models of schizophrenia, the precise role of mitochondrial dynamics in the pathophysiology of schizophrenia is all but unexplored. Here we discuss the influence of mitochondrial dynamics and mitochondrial function on nervous system development, and highlight recent work suggesting a link between aberrant mitochondrial dynamics and schizophrenia.

A systematic meta-analysis of the association of Neuregulin 1 (NRG1), D-amino acid oxidase (DAO), and DAO activator (DAOA)/G72 polymorphisms with schizophrenia

The glutamate hypothesis of schizophrenia is related to the proposed dysregulation of D-amino acid oxidase (DAO), DAO activator (DAOA)/G72, and Neuregulin 1 (NRG1) genes. Genetic studies have shown significant associations between DAO, DAOA, NRG1 single-nucleotide polymorphisms (SNPs), and schizophrenia. The systematic literature search yielded 6, 5, and 18 new studies for DAO, DAOA, and NRG1 published after 2011 and not included in the previous SchizophreniaGene (SZGene) meta-analysis. We conducted meta-analyses of 20, 23, and 48 case-control studies, respectively, to comprehensively evaluate the association of 8 DAO, 12 DAOA, and 14 NRG1 SNPs with schizophrenia. The updated meta-analyses resulted in the following findings: the C-allele of DAO rs4623951 was associated with schizophrenia across all pooled studies [Odds ratio (OR) = 0.88, 95% confidence interval (CI) = 0.79-0.98, p = 0.02, N = 3143]; however, no new reports could be included. The G-allele of DAOA rs778293 was associated with schizophrenia in Asian patients (OR = 1.17, 95% CI = 1.08-1.27, p = 0.00008, N = 6117), and the T-allele of DAOA rs3916971 was associated with schizophrenia across all studies (OR = 0.84, 95% CI = 0.73-0.96, p = 0.01, N = 1765). Again, for both SNPs, no new eligible studies were available. After adding new reports, the T-allele of NRG1 SNP8NRG241930 (rs62510682) across all studies (OR = 0.95, 95% CI = 0.91-0.997, p = 0.04, N = 22,898) and in Caucasian samples (OR = 0.95, 95% CI = 0.90-0.99, p = 0.03, N = 16,014), and the C-allele of NRG1 rs10503929 across all studies (OR = 0.89, 95% CI = 0.81-0.97, p = 0.01, N = 6844) and in Caucasian samples (OR = 0.89, 95% CI = 0.81-0.98, p = 0.01, N = 6414) were protective against schizophrenia. Our systematic meta-analysis is the most updated one for the association of DAO, DAOA, and NRG1 SNPs with schizophrenia.

Plasma and cerebrospinal fluid G72 protein levels in schizophrenia and major depressive disorder

G72 is a modulator of D-amino acid oxidase, the enzyme that degrades D-serine, an amino acid that plays a critical role in glutamate neurotransmission, and has been implicated in psychiatric disorders. The aim of this study was to examine whether plasma or cerebrospinal fluid (CSF) G72 protein levels were altered in either schizophrenia or major depressive disorder (MDD) and whether any correlation between G72 levels and disease severity existed. Initially, 27 schizophrenic patients, 26 MDD patients, and 27 healthy controls matched for age, sex, and ethnicity were enrolled. Compared to those of controls, plasma or CSF G72 levels were not significantly different in patients with schizophrenia or MDD. Although we found a significant positive correlation between plasma G72 levels and a positive symptoms score on the positive and negative syndrome scale (PANSS), this was not replicated in the second study (40 schizophrenic patients). CSF G72 levels showed no significant correlation with PANSS scores. In MDD, neither plasma nor CSF G72 levels correlated significantly with depression severity. Since severity of our patients were relatively mild, further investigations in a large number of subjects including drug-free patients, younger patients, and more severely affected patients are warranted.

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.

Elucidating the role of the pLG72 R30K substitution in schizophrenia susceptibility

In the human brain, pLG72 interacts with the flavoenzyme d-amino acid oxidase (hDAAO), which is involved in catabolism of d-serine, a co-agonist of N-methyl-d-aspartate receptors (NMDAR). Here, we investigated the wild-type pLG72, the R30K variant associated with schizophrenia susceptibility, and the K62E variant. The protein conformation, oligomeric state, ligand-, and hDAAO-binding properties are only slightly modified by the substitutions. All pLG72 variants inhibit hDAAO and lead to an increase in cellular (d/d+l)-serine. However, the R30K pLG72 is significantly more prone to degradation than the R30 and the K62E variants in a cell system, thus possessing a lower ability to interact/inhibit hDAAO. This links R30K pLG72 with the hyperactivity of hDAAO, the decreased d-serine level, and NMDAR hypofunction observed in schizophrenia-affected patients.

Prediction Analysis for Transition to Schizophrenia in Individuals at Clinical High Risk for Psychosis: The Relationship of DAO, DAOA, and NRG1 Variants with Negative Symptoms and Cognitive Deficits

Schizophrenia is characterized by positive and negative symptoms and cognitive dysfunction. The glutamate hypothesis of schizophrenia has been hypothesized to explain the negative symptoms and cognitive deficits better than the dopamine hypothesis alone. Therefore, we aimed to evaluate whether glutamatergic variants such as d-amino acid oxidase (DAO), DAO activator (DAOA)/G72, and neuregulin 1 (NRG1) single-nucleotide polymorphisms (SNPs) and their mRNA levels predicted (i) transition to schizophrenia spectrum disorders and (ii) research domain criteria (RDoC) domains, mainly negative valence and cognitive systems. In a 3-year prospective study cohort of 185 individuals (age: 13-35 years) at high risk and ultra-high risk (UHR) for psychosis, we assessed DAO (rs3918347, rs4623951), DAOA (rs778293, rs3916971, rs746187), and NRG1 (rs10503929) SNPs and their mRNA expression. Furthermore, we investigated their association with RDoC domains, mainly negative valence (e.g., anxiety, hopelessness) and cognitive (e.g., perception disturbances, disorganized symptoms) systems. NRG1 rs10503929 CC + CT versus TT genotype carriers experienced significantly more disorganized symptoms. DAOA rs746187 CC versus CT + TT genotype, DAOA rs3916971 TT versus TC + CC genotype, and DAO rs3918347 GA + AA versus GG genotype carriers experienced nominally more hopelessness, visual perception disturbances, and auditory perception disturbances, respectively. The schizophrenia risk G-allele of DAO rs3918347 nominally increased risk for those UHR individuals with attenuated positive symptoms syndrome. No association between DAO, DAOA, NRG1 SNPs, and conversion to schizophrenia spectrum disorders was observed. Our findings suggest that DAO, DAOA, and NRG1 polymorphisms might influence both RDoC negative valence and cognitive systems, but not transition to schizophrenia spectrum disorders.

Body fluid levels of neuroactive amino acids in autism spectrum disorders: a review of the literature

A review of studies on the body fluid levels of neuroactive amino acids, including glutamate, glutamine, taurine, gamma-aminobutyric acid (GABA), glycine, tryptophan, d-serine, and others, in autism spectrum disorders (ASD) is given. The results reported in the literature are generally inconclusive and contradictory, but there has been considerable variation among the previous studies in terms of factors such as age, gender, number of subjects, intelligence quotient, and psychoactive medication being taken. Future studies should include simultaneous analyses of a large number of amino acids [including d-serine and branched-chain amino acids (BCAAs)] and standardization of the factors mentioned above. It may also be appropriate to use saliva sampling to detect amino acids in ASD patients in the future—this is noninvasive testing that can be done easily more frequently than other sampling, thus providing more dynamic monitoring.