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

L -Amino acids are the building blocks for proteins synthesized in ribosomes in all kingdoms of life, but d -amino acids ( d -aa) have important non-ribosome-based functions 1 . Mammals synthesize d -Ser and d -Asp, primarily in the central nervous system, where d -Ser is critical for neurotransmission 2 . Bacteria synthesize a largely distinct set of d -aa, which become integral components of the cell wall and are also released as free d -aa 3 , 4 . However, the impact of free microbial d -aa on host physiology at the host–microbial interface has not been explored. Here, we show that the mouse intestine is rich in free d -aa that are derived from the microbiota. Furthermore, the microbiota induces production of d -amino acid oxidase (DAO) by intestinal epithelial cells, including goblet cells, which secrete the enzyme into the lumen. Oxidative deamination of intestinal d -aa by DAO, which yields the antimicrobial product H 2 O 2 , protects the mucosal surface in the small intestine from the cholera pathogen. DAO also modifies the composition of the microbiota and is associated with microbial induction of intestinal sIgA. Collectively, these results identify d -aa and DAO as previously unrecognized mediators of microbe–host interplay and homeostasis on the epithelial surface of the small intestine. The mouse gut microbiota produce free d -amino acids and induce the production of d -amino acid oxidase by intestinal epithelial cells. Oxidative deamination of d -amino acids yields H 2 O 2 , which protects the mucosa from Vibrio cholera e.

D-Amino acid oxidase (DAAO) is a FAD-containing flavoenzyme that catalyzes the oxidative deamination of D-isomers of neutral and polar amino acids. This enzymatic activity has been identified in most eukaryotic organisms, the only exception being plants. In the various organisms in which it does occur, DAAO fulfills distinct physiological functions: from a catabolic role in yeast cells, which allows them to grow on D-amino acids as carbon and energy sources, to a regulatory role in the human brain, where it controls the levels of the neuromodulator D-serine. Since 1935, DAAO has been the object of an astonishing number of investigations and has become a model for the dehydrogenase-oxidase class of flavoproteins. Structural and functional studies have suggested that specific physiological functions are implemented through the use of different structural elements that control access to the active site and substrate/product exchange. Current research is attempting to delineate the regulation of DAAO functions in the contest of complex biochemical and physiological networks.

Aims/hypothesisDiabetic retinopathy is characterised by retinal neurodegeneration and retinal vascular abnormalities, affecting one third of diabetic patients with disease duration of more than 10 years. Accumulated evidence suggests that serine racemase (SR) and D-serine are correlated with the pathogenesis of diabetic retinopathy and the deletion of the Srr gene reverses neurovascular pathologies in diabetic mice. Since D-serine content is balanced by SR synthesis and D-amino acid oxidase (DAAO) degradation, we examined the roles of DAAO in diabetic retinopathy and further explored relevant therapy.MethodsRats were used as a model of diabetes by i.p. injection of streptozotocin at the age of 2 months and blood glucose was monitored with a glucometer. Quantitative real-time PCR was used to examine Dao mRNA and western blotting to examine targeted proteins in the retinas. Bisulphite sequencing was used to examine the methylation of Dao mRNA promoter in the retinas. Intravitreal injection of DAAO-expressing adenovirus (AAV8-DAAO) was conducted one week before streptozotocin administration. Brain specific homeobox/POU domain protein 3a (Brn3a) immunofluorescence was conducted to indicate retinal ganglion cells at 3 months after virus injection. The permeability of the blood–retinal barrier was examined by Evans blue leakage from retinal capillaries. Periodic acid–Schiff staining and haematoxylin counterstaining were used to indicate retinal vasculature, which was further examined with double immunostaining at 7 months after virus injection.ResultsAt the age of 12 months, DAAO mRNA and protein levels in retinas from diabetic animals were reduced to 66.2% and 70.4% of those from normal (control) animals, respectively. The Dao proximal promoter contained higher levels of methylation in diabetic than in normal retinas. Consistent with the observation, DNA methyltransferase 1 was increased in diabetic retinas. Injection of DAAO-expressing virus completely prevented the loss of retinal ganglion cells and the disruption of blood–retinal barrier in diabetic rats. Diabetic retinas contained retinal ganglion cells at a density of 54 ± 4/mm2, which was restored to 68 ± 9/mm2 by DAAO overexpression, similar to the levels in normal retinas. The ratio between the number of endothelial cells and pericytes in diabetic retinas was 6.06 ± 1.93/mm2, which was reduced to 3.42 ± 0.55/mm2 by DAAO overexpression; the number of acellular capillaries in diabetic retinas was 10 ± 5/mm2, which was restored to 6 ± 2/mm2 by DAAO overexpression, similar to the levels in normal retinas. Injection of the DAAO-expressing virus increased the expression of occludin and reduced gliosis, which were examined to probe the mechanism by which the disrupted blood–retinal barrier in diabetic rats was rescued and retinal neurodegeneration was prevented.Conclusions/interpretationAltogether, overexpression of DAAO before the onset of diabetes protects against neurovascular abnormalities in retinas from diabetic rats, which suggests a novel strategy for preventing diabetic retinopathy.

Arsenic exposure model of offspring mice was established and intervened with 6-chlorobenzo[d]isoxazol-3-ol (CBIO), a D-amino acid oxidase (DAAO) inhibitor, to explore the role of DAAO in cognitive impairment of offspring mice induced by arsenic during early developmental stage. Female mice and their pups treated with 0 or 60 mg/L sodium arsenite (NaAsO 2 ) via drinkable water from the first day of gestation till the end of lactation. On the 28th day after birth, the offspring mice in the drinking distilled water group were randomly divided into control and 1 mg/mL CBIO group. The offspring mice in the arsenic group were divided into 60 mg/L NaAsO 2 group and 60 mg/L NaAsO 2  + 1 mg/mL CBIO group, CBIO was administered to the lateral ventricle for one week. Additionally, D-serine and L-serine concentrations were detected by UHPLC-MS/MS, Real-time RT-PCR and Western blot were applied to measure DAAO, serine racemase (SR), N-methyl-D-aspartate receptor (NMDAR), synaptophysin (SYP) and postsynaptic density (PSD95) levels in the hippocampus. Results disclosed that arsenic could reduce the levels of D-serine, L-serine, SR and NMDAR, while upregulate DAAO levels, however, inhibiting DAAO levels could increase D-serine and NR1 levels. These findings indicated that DAAO might be involved in cognitive impairment of offspring mice induced by arsenic during early developmental stage by affecting D-serine metabolism.

Background Patient-derived organoids provide a powerful platform for elucidating mechanisms of drug resistance and tumor evolution in hepatocellular carcinoma (HCC) and identifying novel therapeutic targets. Methods Transcriptomic sequencing was used to compare gene expression patterns between organoid-forming and non-forming HCC tissues, as well as between sorafenib-resistant organoids and sorafenib-sensitive counterparts. The TCGA-liver hepatocellular carcinoma (LIHC) cohort was used to screen for the key molecular drivers of HCC evolution from the overlapping differentially expressed genes. The effects of D-amino acid oxidase (DAO) on the growth, migration, invasion, apoptosis, and generation of H 2 O 2 were evaluated in HepG2 and SK-Hep-1 cell lines and human HCC organoids. The therapeutic efficacy of DAO against HCC growth and drug resistance was validated with xenograft mouse model and organoids, respectively. Results Expression level of DAO was significantly downregulated in HCC tissues that successfully formed organoids compared to those that failed, as well as in sorafenib-resistant organoids versus their parental counterparts. In the TCGA-LIHC cohort, DAO expression was significantly reduced in advanced-stage HCC tissues and was inversely correlated with stemness- and epithelial-mesenchymal transition (EMT)-related molecules. Lower DAO expression was associated with poor overall survival in patients with HCC. In HepG2 cells, DAO knockdown significantly enhanced cell proliferation. Ectopic DAO expression suppressed proliferation, migration, and invasion in HepG2 and SK-Hep-1 cells. D-alanine (D-Ala) supplementation further enhanced the anti-proliferative effect of overexpressed DAO, but did not significantly alter the DAO-mediated suppression of migration or invasion. Ectopic expression of DAO induced apoptosis via the generation of H 2 O 2 upon simultaneous supplementation of D-Ala into the culture medium; the addition of catalase, an H₂O₂-degrading enzyme, significantly reversed the D-Ala-induced effects. In BALB/c nude mouse models, HCC cells overexpressing DAO formed significantly smaller tumors than the control cells ( P  = 0.010), and this tumor-suppressive effect was further enhanced by D-alanine supplementation. Ectopic DAO expression restored sorafenib sensitivity in resistant organoids. Conclusion DAO appears to be a novel endogenous stemness repressor. The reduction in DAO is a critical molecular event in the evolution of HCC. Therapeutically, combined DAO and D-amino acid supplementation is a promising strategy for HCC treatment, particularly for reversing sorafenib resistance.

Free d-amino acids, which are enantiomers of l-amino acids, are found in mammals, including humans, and play an important role in a range of physiological functions in the central nervous system and peripheral tissues. Several d-amino acids have been observed in saliva, but their origin and the enzymes involved in their metabolism and catabolism remain to be clarified. In the present study, large amounts of d-aspartic acid and small amounts of d-serine and d-alanine were detected in all three major salivary glands in rat. No other d-enantiomers were detected. Protein expression of d-amino acid oxidase and d-aspartate oxidase, the enzymes responsible for the oxidative deamination of neutral and dicarboxylic d-amino acids, respectively, were detected in all three types of salivary gland. Furthermore, protein expression of the d-serine metabolic enzyme, serine racemase, in parotid glands amounted to approximately 40% of that observed in the cerebral cortex. The N-methyl-d-aspartic acid subunit proteins NR1 and NR2D were detected in all three major salivary glands. The results of the present study suggest that d-amino acids play a physiological role in a range of endocrine and exocrine function in salivary glands.

Since d -amino acids were identified in mammals, d -serine has been one of the most extensively studied “unnatural amino acids”. This brain-enriched transmitter-like molecule plays a pivotal role in the human central nervous system by modulating the activity of NMDA receptors. Physiological levels of d -serine are required for normal brain development and function; thus, any alterations in neuromodulator concentrations might result in NMDA receptor dysfunction, which is known to be involved in several pathological conditions, including neurodegeneration(s), epilepsy, schizophrenia, and bipolar disorder. In the brain, the concentration of d -serine stored in cells is defined by the activity of two enzymes: serine racemase (responsible for both the synthesis and degradation) and d -amino acid oxidase (which catalyzes d -serine degradation). Both enzymes emerged recently as new potential therapeutic targets for NMDA receptor-related diseases. In this review we have focused on human d -amino acid oxidase and provide an extensive overview of the biochemical and structural properties of this flavoprotein and their functional significance. Furthermore, we discuss the mechanisms involved in modulating enzyme activity and stability with the aim to substantiate the pivotal role of d -amino acid oxidase in brain d -serine metabolism in physiological and pathological conditions and to highlight its great significance for novel drug design/development.

Abstract Background Current anti-dementia drugs cannot benefit mild cognitive impairment (MCI). Sodium benzoate (a D-amino acid oxidase [DAO] inhibitor) has been found to improve the cognitive function of patients with early-phase Alzheimer’s disease (mild Alzheimer’s disease or MCI). However, its effect on brain function remains unknown. This study aimed to evaluate the influence of benzoate on functional magnetic resonance imaging in patients with amnestic MCI. Methods This was a 24-week, randomized, double-blind, placebo-controlled trial that enrolled 21 patients with amnestic MCI and allocated them randomly to either of 2 treatment groups: (1) benzoate group (250–1500 mg/d), or (2) placebo group. We assessed the patients’ working memory, verbal learning and memory, and resting-state functional magnetic resonance imaging and regional homogeneity (ReHo) maps at baseline and endpoint. Results Resting-state ReHo decreased in right orbitofrontal cortex after benzoate treatment but did not change after placebo. Moreover, after benzoate treatment, the change in working memory was positively correlated with the change in ReHo in right precentral gyrus and right middle occipital gyrus; and the change in verbal learning and memory was positively correlated with the change in ReHo in left precuneus. In contrast, after placebo treatment, the change in working memory or in verbal learning and memory was not correlated with the change in ReHo in any brain region. Conclusion The current study is the first to our knowledge to demonstrate that a DAO inhibitor, sodium benzoate herein, can alter brain activity as well as cognitive functions in individuals with MCI. The preliminary finding lends supports for DAO inhibition as a novel approach for early dementing processes.

d- Amino acid oxidase (DAAO) is a valuable flavoenzyme capable of being used in various practical applications, such as in determining d -amino acids and producing a material for semisynthetic cephalosporins, requiring higher thermal stability, higher catalytic activity, and broad substrate specificity. In this study, we isolated the thermophilic fungus Rasamsonia emersonii strain YA, which can grow on several d -amino acids as the sole nitrogen source, from a compost and characterized DAAO (ReDAAO) of the fungus. ReDAAO expressed in Escherichia coli exhibited significant oxidase activity against various neutral and basic d- amino acids, in particular hydrophobic d -amino acids. In addition, the enzyme also significantly acted on cephalosporin C, a starting material for semisynthetic antibiotics, and d- Glu, a general substrate for d- aspartate oxidase but not for DAAO, showing its unique and practically useful substrate specificity. The apparent k cat and K m values of the enzyme toward good substrates were comparable to those of higher catalytic fungal DAAOs, and the thermal stability ( T 50 value of ~60 °C) was comparable to that of a thermophilic bacterial DAAO and significantly higher than that of other eukaryotic DAAOs. These results highlight the great potential of ReDAAO for use in practical applications.