Expression of the G72/G30 gene in transgenic mice induces behavioral changes

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.

DNA methylation patterns of FKBP5 regulatory regions in brain and blood of humanized mice and humans

Humanized mouse models can be used to explore human gene regulatory elements (REs), which frequently lie in non-coding and less conserved genomic regions. Epigenetic modifications of gene REs, also in the context of gene x environment interactions, have not yet been explored in humanized mouse models. We applied high-accuracy measurement of DNA methylation (DNAm) via targeted bisulfite sequencing (HAM-TBS) to investigate DNAm in three tissues/brain regions (blood, prefrontal cortex and hippocampus) of mice carrying the human FK506-binding protein 5 (FKBP5) gene, an important candidate gene associated with stress-related psychiatric disorders. We explored DNAm in three functional intronic glucocorticoid-responsive elements (at introns 2, 5, and 7) of FKBP5 at baseline, in cases of differing genotype (rs1360780 single nucleotide polymorphism), and following application of the synthetic glucocorticoid dexamethasone. We compared DNAm patterns in the humanized mouse (N = 58) to those in human peripheral blood (N = 447 and N = 89) and human postmortem brain prefrontal cortex (N = 86). Overall, DNAm patterns in the humanized mouse model seem to recapitulate DNAm patterns observed in human tissue. At baseline, this was to a higher extent in brain tissue. The animal model also recapitulated effects of dexamethasone on DNAm, especially in peripheral blood and to a lesser extent effects of genotype on DNAm. The humanized mouse model could thus assist in reverse translation of human findings in psychiatry that involve genetic and epigenetic regulation in non-coding elements.

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.

Behavioral Tasks Evaluating Schizophrenia-like Symptoms in Animal Models: A Recent Update

BACKGROUND

Schizophrenia is a serious mental illness that affects more than 21 million people worldwide. Both genetics and the environment play a role in its etiology and pathogenesis. Symptoms of schizophrenia are mainly categorized into positive, negative, and cognitive. One major approach to identify and understand these diverse symptoms in humans has been to study behavioral phenotypes in a range of animal models of schizophrenia.

OBJECTIVE

We aimed to provide a comprehensive review of the behavioral tasks commonly used for measuring schizophrenia-like behaviors in rodents together with an update of the recent study findings.

METHODS

Articles describing phenotypes of schizophrenia-like behaviors in various animal models were collected through a literature search in Google Scholar, PubMed, Web of Science, and Scopus, with a focus on advances over the last 10 years.

RESULTS

Numerous studies have used a range of animal models and behavioral paradigms of schizophrenia to develop antipsychotic drugs for improved therapeutics. In establishing animal models of schizophrenia, the candidate models were evaluated for schizophrenia-like behaviors using several behavioral tasks for positive, negative, and cognitive symptoms designed to verify human symptoms of schizophrenia. Such validated animal models were provided as rapid preclinical avenues for drug testing and mechanistic studies.

CONCLUSION

Based on the most recent advances in the field, it is apparent that a myriad of behavior tests are needed to confirm and evaluate the congruency of animal models with the numerous behaviors and clinical signs exhibited by patients with schizophrenia.

pLG72 induces superoxide radicals via interaction and aggregation with SOD1

G72 has been characterised as a susceptibility gene that can have wide-ranging effects in a number of neurodegenerative diseases, including schizophrenia and major depression. Indeed, its product, pLG72, is a potential serum biomarker for schizophrenia. Previous transcriptomic and biochemical studies have indicated that pLG72 may induce the production of mitochondrial reactive oxygen species (ROS), resulting in cell damage. Here, we investigated the mechanism of pLG72 by transfecting a human U87 glioblastoma cell line with a G72 construct. By employing ROS-specific scavengers, we discovered that superoxide radicals were specifically induced in the pLG72-expressing cells. We also found that pLG72 interacted and co-localised with superoxide dismutase 1 (SOD1), resulting in aggregation of SOD1 with a concomitant 23% or 74% reduction of total SOD activity, depending on the amount of G72 transfection plasmid. Finally, we found that transfection of U87 cells with the G72 construct caused a 29% decrease in cell proliferation. The observed loss of SOD1 function in pLG72-expressing cells may explain the elevated ROS levels and inhibition of U87 cell proliferation and has implications for understanding the onset of neurodegenerative diseases in humans.

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.

Combination of G72 Genetic Variation and G72 Protein Level to Detect Schizophrenia: Machine Learning Approaches

The D-amino acid oxidase activator (DAOA, also known as G72) gene is a strong schizophrenia susceptibility gene. Higher G72 protein levels have been implicated in patients with schizophrenia. The current study aimed to differentiate patients with schizophrenia from healthy individuals using G72 single nucleotide polymorphisms (SNPs) and G72 protein levels by leveraging computational artificial intelligence and machine learning tools. A total of 149 subjects with 89 patients with schizophrenia and 60 healthy controls were recruited. Two G72 genotypes (including rs1421292 and rs2391191) and G72 protein levels were measured with the peripheral blood. We utilized three machine learning algorithms (including logistic regression, naive Bayes, and C4.5 decision tree) to build the optimal predictive model for distinguishing schizophrenia patients from healthy controls. The naive Bayes model using two factors, including G72 rs1421292 and G72 protein, appeared to be the best model for disease susceptibility (sensitivity = 0.7969, specificity = 0.9372, area under the receiver operating characteristic curve (AUC) = 0.9356). However, a model integrating G72 rs1421292 only slightly increased the discriminative power than a model with G72 protein alone (sensitivity = 0.7941, specificity = 0.9503, AUC = 0.9324). Among the three models with G72 protein alone, the naive Bayes with G72 protein alone had the best specificity (0.9503), while logistic regression with G72 protein alone was the most sensitive (0.8765). The findings remained similar after adjusting for age and gender. This study suggests that G72 protein alone, without incorporating the two G72 SNPs, may have been suitable enough to identify schizophrenia patients. We also recommend applying both naive Bayes and logistic regression models for the best specificity and sensitivity, respectively. Larger-scale studies are warranted to confirm the findings.

Expression of D-Amino Acid Oxidase (DAO/DAAO) and D-Amino Acid Oxidase Activator (DAOA/G72) during Development and Aging in the Human Post-mortem Brain

In the brain, D-amino acid oxidase (DAO/DAAO) mainly oxidizes D-serine, a co-agonist of the N-methyl-D-aspartate (NMDA) receptors. Thus, DAO can regulate the function of NMDA receptors via D-serine breakdown. Furthermore, DAO activator (DAOA)/G72 has been reported as both DAOA and repressor. The co-expression of DAO and DAOA genes and proteins in the human brain is not yet elucidated. The aim of this study was to understand the regional and age span distribution of DAO and DAOA (mRNA and protein) in a concomitant manner. We determined DAO and DAOA mRNA and protein expression across six brain regions in normal human post-mortem brain samples (16 weeks of gestation to 91 years) using quantitative real-time reverse transcription-polymerase chain reaction and enzyme-linked immunosorbent assay. We found higher expression of DAO mRNA in the cerebellum, whereas lower expression of DAO protein in the cerebellum compared to the other brain regions studied, which suggests post-transcriptional regulation. We detected DAOA protein but not DAOA mRNA in all brain regions studied, suggesting a tightly regulated expression. To understand this regulation at the transcriptional level, we analyzed DNA methylation levels at DAO and DAOA CpG sites in the cerebellum and frontal cortex of control human post-mortem brain obtained from Gene Expression Omnibus datasets. Indeed, DAO and DAOA CpG sites in the cerebellum were significantly more methylated than those in the frontal cortex. While investigating lifespan effects, we found that DAO mRNA levels were positively correlated with age <2 years in the cerebellum and amygdala. We also detected a significant positive correlation (controlled for age) between DAO and DAOA protein in all of the brain regions studied except for the frontal cortex. In summary, DAO and DAOA expression in the human brain are both age and brain region dependent.

Pharmacoinformatics, Adaptive Evolution, and Elucidation of Six Novel Compounds for Schizophrenia Treatment by Targeting DAOA (G72) Isoforms

Studies on Schizophrenia so far reveal a complex picture of neurological malfunctioning reported to be strongly associated with DAOA. Detailed sequence analyses proved DAOA as a primate specific gene having conserved gene desert region on both upstream and downstream region. The analyses of 10 MB chromosomal region of primates, birds, rodents, and reptiles having DAOA evidenced the conserved part in primates and in the rest of species, while DAOA is only present in primates. DAOA has four isoforms having one interaction partner DAO. Protein-protein analyses of four DAOA isoforms with DAO were performed individually and find potential interacting residues computationally. It was observed that molecular docking of approved FDA drugs revealed efficient results but there was no common drug with effective binding to all DAOA isoforms. Library of compounds was constructed by virtual screening of 2D similarity search against recommended SZ drugs in conjunction with their physiochemical properties. Molecular docking resulted in six novel compounds exhibiting maximum binding affinity with selected four DAOA isoforms. However not the entire schizophrenic population responds to the single drug and interestingly in this study six novel compounds having promising results and same binding site to that DAOA that may be used to interact with DAO against four DAOA isoforms were observed.

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.

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

Numerous studies have demonstrated a link between genetic markers on chromosome 13 and schizophrenia, bipolar affective disorder, and other psychiatric phenotypes. The G72/G30 genes (transcribed in opposite directions) are located on chromosome 13q33, a region demonstrating strong evidence for linkage with various neuropsychiatric disorders. G72/G30 was identified in 2002 as a schizophrenia susceptibility locus; however, subsequent association studies did not reach consensus on single SNPs within the locus. Simultaneously, a new vision for the genetic architecture of psychiatric disorders suggested that schizophrenia was a quantitative trait, therefore ascribable to potentially hundreds of genes and subjected to the vagaries of the environment. The main protein product of G72 gene is named pLG72 or D-amino acid oxidase activator DAOA (153 amino acids) and its function is still debated. Functional analyses, also showing controversial results, indicate that pLG72 contributes to N-methyl-D-aspartate receptor modulation by affecting activity of the flavoprotein D-amino acid oxidase, the enzyme responsible for degrading the neuromodulator D-serine. In this review we, for the first time, summarize findings from molecular genetic linkage and association studies concerning G72 gene, cellular and molecular studies on pLG72, and investigations performed on G72/G30 transgenic mice. This will help elucidate the role of psychosis susceptibility genes, which will have a major impact on our understanding of disease pathophysiology and thus change classification and treatment.

A Mutation in DAOA Modifies the Age of Onset in PSEN1 E280A Alzheimer's Disease

We previously reported age of onset (AOO) modifier genes in the world's largest pedigree segregating early-onset Alzheimer's disease (AD), caused by the p.Glu280Ala (E280A) mutation in the PSEN1 gene. Here we report the results of a targeted analysis of functional exonic variants in those AOO modifier genes in sixty individuals with PSEN1 E280A AD who were whole-exome genotyped for ~250,000 variants. Standard quality control, filtering, and annotation for functional variants were applied, and common functional variants located in those previously reported as AOO modifier loci were selected. Multiloci linear mixed-effects models were used to test the association between these variants and AOO. An exonic missense mutation in the G72 (DAOA) gene (rs2391191, P = 1.94 × 10⁻⁴, P_FDR = 9.34 × 10⁻³) was found to modify AOO in PSEN1 E280A AD. Nominal associations of missense mutations in the CLUAP1 (rs9790, P = 7.63 × 10⁻³, P_FDR = 0.1832) and EXOC2 (rs17136239, P = 0.0325, P_FDR = 0.391) genes were also found. Previous studies have linked polymorphisms in the DAOA gene with the occurrence of neuropsychiatric symptoms such as depression, apathy, aggression, delusions, hallucinations, and psychosis in AD. Our findings strongly suggest that this new conspicuous functional AOO modifier within the G72 (DAOA) gene could be pivotal for understanding the genetic basis of AD.

Identification of pLG72-Induced Oxidative Stress Using Systemic Approaches

G72 is a schizophrenia-susceptible gene encoding a polypeptide with 153 amino acids. In 2002, it was originally proposed as an activator of D-amino acid oxidase (DAOA) that could enhance the activity of DAAO and subsequently reduce the neurotransmission of N-methyl-D-aspartate receptors. However, several controversial findings have been reported recently. Due to a number of inconsistent descriptions of pLG72's biofunctions, this study aims to identify the cellular effects induced by pLG72 in U87 cells using systems biology approaches. The analyses of transcriptomics and biological networks showed that pLG72 might be involved in the induction of oxidative stress. To confirm the in silico prediction, we tested and discovered that overexpression of pLG72 effectively enhanced reactive oxygen species (ROS) in U87 cells and, furthermore, this induction can be quenched by Tempol, a general ROS scavenger. Therefore, G72-transgenic mice presenting some psychiatric symptoms, along with the pLG72 level being significantly increased in the serum of patients with schizophrenia, have led us to propose that the ROS enhancement in mental diseases may be from the overexpression of pLG72 in brain cells.

Adaptive evolution and elucidating the potential inhibitor against schizophrenia to target DAOA (G72) isoforms

Schizophrenia (SZ), a chronic mental and heritable disorder characterized by neurophysiological impairment and neuropsychological abnormalities, is strongly associated with D-amino acid oxidase activator (DAOA, G72). Research studies emphasized that overexpression of DAOA may be responsible for improper functioning of neurotransmitters, resulting in neurological disorders like SZ. In the present study, a hybrid approach of comparative modeling and molecular docking followed by inhibitor identification and structure modeling was employed. Screening was performed by two-dimensional similarity search against selected inhibitor, keeping in view the physiochemical properties of the inhibitor. Here, we report an inhibitor compound which showed maximum binding affinity against four selected isoforms of DAOA. Docking studies revealed that Glu-53, Thr-54, Lys-58, Val-85, Ser-86, Tyr-87, Leu-88, Glu-90, Leu-95, Val-98, Ser-100, Glu-112, Tyr-116, Lys-120, Asp-121, and Arg-122 are critical residues for receptor–ligand interaction. The C-terminal of selected isoforms is conserved, and binding was observed on the conserved region of isoforms. We propose that selected inhibitor might be more potent on the basis of binding energy values. Further analysis of this inhibitor through site-directed mutagenesis could be helpful for exploring the details of ligand-binding pockets. Overall, the findings of this study may be helpful in designing novel therapeutic targets to cure SZ.

Genetic variation in the G72 gene is associated with increased frontotemporal fiber tract integrity

G72 (syn. DAOA, D-amino acid oxidase activator) is a susceptibility gene for both schizophrenia and bipolar disorder. Diffusion tensor imaging studies hint at changes in fiber tract integrity in both disorders. We aimed to investigate whether a G72 susceptibility haplotype causes changes in fiber tract integrity in young healthy subjects. We compared fractional anisotropy in 47 subjects that were either homozygous for the M23/M24 risk haplotype (n = 20) or homozygous for M23(rs3918342)/M24(rs1421292) wild type (n = 27) using diffusion tensor imaging with 3 T. Tract-based spatial statistics, a method especially developed for diffusion data analysis, was used to delineate the major fiber tracts. We found clusters of increased FA values in homozygous risk haplotype carriers in the right periinsular region and in the right inferior parietal lobe (IPL). We did not find clusters indicating decreased FA values. The insula and the IPL have been implicated in both schizophrenia and bipolar pathophysiology. Increased FA values might reflect changes in dendritic morphology as previously described by in vitro studies. These findings further corroborate the hypothesis that a shared gene pool between schizophrenia and bipolar disorder might lead to neuroanatomic changes that confer an unspecific vulnerability for both disorders.

Chronic nicotine improves short-term memory selectively in a G72 mouse model of schizophrenia

BACKGROUND AND PURPOSE

The prevalence of smoking in schizophrenia patients is exceptionally high; it is not known why but many researchers suggest that smoking constitutes a form of self-medication. Among the symptoms of schizophrenia that may be improved by nicotine are cognitive deficits. Hence, we studied the effects of long-term nicotine administration on cognition in a genetic animal model of schizophrenia susceptibility, G72-transgenic (G72Tg) mice.

EXPERIMENTAL APPROACH

The effect of long-term nicotine or saline, administered by osmotic minipumps, on different cognitive domains was assessed in G72Tg mice and controls using a battery of behavioural tests. To investigate the mechanism underlying phenotypic differences, quantitative autoradiographic mapping of nACh receptor subtypes was performed in forebrain structures to explore effects of chronic nicotine exposure on nACh receptor density in wild-type (WT) and G72Tg mice.

KEY RESULTS

Genotype significantly affected the cognitive effects of chronic nicotine administration. Whereas chronic nicotine disrupted cognitive performance in WT mice, it was effective at restoring impaired prepulse inhibition, working memory and social recognition in G72Tg mice. However, long-term spatial learning was further impaired by nicotine in transgenic animals. In contrast, associative learning was protected by G72-expression against the adverse nicotine effects seen in WT animals. G72-expression did not decisively influence nicotine-induced up-regulation of the α4β2*subtype, whereas α7nACh receptor density was differentially altered by genotype or by a genotype·treatment interaction in specific brain areas, most notably hippocampal subregions.

CONCLUSIONS AND IMPLICATIONS

Our data support the hypothesis that nicotine self-medication of schizophrenics improves cognitive symptoms, possibly by facilitating nicotine-induced α7nACh receptor activation in the hippocampus.

Pre-clinical models of neurodevelopmental disorders: focus on the cerebellum

Recent studies have advanced our understanding of the role of the cerebellum in non-motor behaviors. Abnormalities in the cerebellar structure have been demonstrated to produce changes in emotional, cognitive, and social behaviors resembling clinical manifestations observed in patients with autism spectrum disorders (ASD) and schizophrenia. Several animal models have been used to evaluate the effects of relevant environmental and genetic risk factors on the cerebellum development and function. However, very few models of ASD and schizophrenia selectively target the cerebellum and/or specific cell types within this structure. In this review, we critically evaluate the strength and weaknesses of these models. We will propose that the future progress in this field will require time- and cell type-specific manipulations of disease-relevant genes, not only selectively in the cerebellum, but also in frontal brain areas connected with the cerebellum. Such information can advance our knowledge of the cerebellar contribution to non-motor behaviors in mental health and disease.

The C-terminal region of G72 increases D-amino acid oxidase activity

The schizophrenia-related protein G72 plays a unique role in the regulation of d-amino acid oxidase (DAO) in great apes. Several psychiatric diseases, including schizophrenia and bipolar disorder, are linked to overexpression of DAO and G72. Whether G72 plays a positive or negative regulatory role in DAO activity, however, has been controversial. Exploring the molecular basis of the relationship between G72 and DAO is thus important to understand how G72 regulates DAO activity. We performed yeast two-hybrid experiments and determined enzymatic activity to identify potential sites in G72 involved in binding DAO. Our results demonstrate that residues 123–153 and 138–153 in the long isoform of G72 bind to DAO and enhance its activity by 22% and 32%, respectively. A docking exercise indicated that these G72 peptides can interact with loops in DAO that abut the entrance of the tunnel that substrate and cofactor must traverse to reach the active site. We propose that a unique gating mechanism underlies the ability of G72 to increase the activity of DAO. Because upregulation of DAO activity decreases d-serine levels, which may lead to psychiatric abnormalities, our results suggest a molecular mechanism involving interaction between DAO and the C-terminal region of G72 that can regulate N-methyl-d-aspartate receptor-mediated neurotransmission.