Simultaneous two-dimensional HPLC determination of free d-serine and d-alanine in the brain and periphery of mutant rats lacking d-amino-acid oxidase

Development of a three-dimensional HPLC system for the determination of serine, threonine and allo-threonine enantiomers in the plasma of patients with chronic kidney disease

A highly-selective three-dimensional high-performance liquid chromatographic (3D-HPLC) system was developed for the determination of serine (Ser), threonine (Thr) and allo-threonine (aThr) enantiomers in human plasma to screen the new biomarker of chronic kidney disease (CKD). d-Ser has been reported to be the candidate biomarker of CKD, however, multiple biomarkers are still required. Therefore, Ser analogs of hydroxy amino acids are the focus in the present study. For the sensitive analysis, the amino acids were derivatized with 4-fluoro-7-nitro-2,1,3-benzoxadiazole and detected by their fluorescence. The 3D-HPLC system consisted of a reversed-phase column (Singularity RP18, 1.0 × 250 mm), an anion-exchange column (Singularity AX, 1.0 × 150 mm) and a Pirkle-type chiral stationary phase (Singularity CSP-013S, 1.5 × 250 mm). The developed method was validated and applied to the human plasma samples obtained from 15 healthy volunteers and 165 CKD patients. The concentrations of the d-forms were 1.13-2.26 (Ser), 0.01-0.03 (Thr) and 0.04-0.10 μM (aThr) for the healthy volunteers and 0.95-19.0 (Ser), 0-0.57 (Thr) and 0.04-1.02 μM (aThr) for the CKD patients. The concentrations and the %d values of all the target d-amino acids were increased along with the decreasing of renal function and further investigation for clinical applications are expected.

D-Amino acids differentially trigger an inflammatory environment in vitro

Studies in vivo have demonstrated that the accumulation of D-amino acids (D-AAs) is associated with age-related diseases and increased immune activation. However, the underlying mechanism(s) of these observations are not well defined. The metabolism of D-AAs by D-amino oxidase (DAO) produces hydrogen peroxide (H2O2), a reactive oxygen species involved in several physiological processes including immune response, cell differentiation, and proliferation. Excessive levels of H2O2 contribute to oxidative stress and eventual cell death, a characteristic of age-related pathology. Here, we explored the molecular mechanisms of D-serine (D-Ser) and D-alanine (D-Ala) in human liver cancer cells, HepG2, with a focus on the production of H2O2 the downstream secretion of pro-inflammatory cytokine and chemokine, and subsequent cell death. In HepG2 cells, we demonstrated that D-Ser decreased H2O2 production and induced concentration-dependent depolarization of mitochondrial membrane potential (MMP). This was associated with the upregulation of activated NF-кB, pro-inflammatory cytokine, TNF-α, and chemokine, IL-8 secretion, and subsequent apoptosis. Conversely, D-Ala-treated cells induced H2O2 production, and were also accompanied by the upregulation of activated NF-кB, TNF-α, and IL-8, but did not cause significant apoptosis. The present study confirms the role of both D-Ser and D-Ala in inducing inflammatory responses, but each via unique activation pathways. This response was associated with apoptotic cell death only with D-Ser. Further research is required to gain a better understanding of the mechanisms underlying D-AA-induced inflammation and its downstream consequences, especially in the context of aging given the wide detection of these entities in systemic circulation.

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

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

Association Between Risperidone Use and Kidney Function Decline in Patients with Schizophrenia: A Retrospective Cohort Study

PURPOSE

Several D-amino acids have been shown to be protective against kidney injury in mice. Risperidone, a currently used atypical antipsychotic agent for schizophrenia, is also known to inhibit the activity of D-amino acid oxidase, which degrades certain D-amino acids. Based on the hypothesis that risperidone would prevent kidney disease progression, this study investigated the association between risperidone use and kidney function decline in patients with schizophrenia.

METHODS

This retrospective cohort study included patients who were diagnosed with schizophrenia and had data available from two or more serum creatinine measurements between April 1, 2010, and March 31, 2020. Patients who used risperidone for at least 30 days were included in the risperidone group, whereas those who had no record of risperidone use were included in the control group. Cox regression models were used to evaluate the risk for 40% decline in estimated glomerular filtration rate (eGFR) in patients treated with risperidone compared to that in the control group.

FINDINGS

Overall, 212 patients used risperidone and 1468 patients had no record of risperidone use. The mean age was 55 years, 759 (45%) of the patients were male, and the mean eGFR at baseline was 88 mL/min/1.73 m2. The mean age in the risperidone group was less than that in the control group (52 vs 56 years); other baseline characteristics were comparable between the two groups. During a mean follow-up of 1.6 years, 267 patients (16%) had a 40% eGFR decline. The incidence rate of 40% eGFR decline was lower in the risperidone group than in the control group (60 vs 104 per 1000 person-years). After adjustment for baseline age, sex, and eGFR, risperidone use was associated with a decreased risk for 40% eGFR decline (hazard ratio = 0.54; 95% CI, 0.33-0.87; P = 0.01).

IMPLICATIONS

Risperidone use may be associated with decreased risk for kidney function decline in patients with schizophrenia. Further studies are warranted to validate these findings.

d-Amino acids in biological systems

Investigations on the occurrence and biochemical roles of free D-amino acids and D-amino acid-containing peptides and proteins in living systems have increased in frequency and significance. Their occurrence and roles may vary substantially with progression from microbiotic to evermore advanced macrobiotic systems. We now understand many of the biosynthetic and regulatory pathways, which are outlined herein. Important uses for D-amino acids in plants, invertebrates, and vertebrates are reviewed. Given its importance, a separate section on the occurrence and role of D-amino acids in human disease is presented.

Mammals sustain amino acid homochirality against chiral conversion by symbiotic microbes

Mammals exhibit systemic homochirality of amino acids in L-configurations. While ribosomal protein synthesis requires rigorous chiral selection for L-amino acids, both endogenous and microbial enzymes convert diverse L-amino acids to D-configurations in mammals. However, it is not clear how mammals manage such diverse D-enantiomers. Here, we show that mammals sustain systemic stereo dominance of L-amino acids through both enzymatic degradation and excretion of D-amino acids. Multidimensional high performance liquidchromatography analyses revealed that in blood, humans and mice maintain D-amino acids at less than several percent of the corresponding L-enantiomers, while D-amino acids comprise ten to fifty percent of the L-enantiomers in urine and feces. Germ-free experiments showed that vast majority of D-amino acids, except for D-serine, detected in mice are of microbial origin. Experiments involving mice that lack enzymatic activity to catabolize D-amino acids showed that catabolism is central to the elimination of diverse microbial D-amino acids, whereas excretion into urine is of minor importance under physiological conditions. Such active regulation of amino acid homochirality depends on maternal catabolism during the prenatal period, which switches developmentally to juvenile catabolism along with the growth of symbiotic microbes after birth. Thus, microbial symbiosis largely disturbs homochirality of amino acids in mice, whereas active host catabolism of microbial D-amino acids maintains systemic predominance of L-amino acids. Our findings provide fundamental insight into how the chiral balance of amino acids is governed in mammals and further expand the understanding of interdomain molecular homeostasis in host-microbial symbiosis.

Determination of D-serine and D-alanine Tissue Levels in the Prefrontal Cortex and Hippocampus of Rats After a Single Dose of Sodium Benzoate, a D-Amino Acid Oxidase Inhibitor, with Potential Antipsychotic and Antidepressant Properties

The effects of the N-methyl-D-aspartate receptor activators D-serine, D-alanine, and sarcosine against schizophrenia and depression are promising. Nevertheless, high doses of D-serine and sarcosine are associated with undesirable nephrotoxicity or worsened prostatic cancer. Thus, alternatives are needed. DAAO inhibition can increase D-serine as well as D-alanine and protect against D-serine-induced nephrotoxicity. Although several DAAO inhibitors improve the symptoms of schizophrenia and depression, they can increase the plasma levels but not brain levels of D-serine. The mechanism of action of DAAO inhibitors remains unclear. We investigated the effects of the DAAO inhibitor sodium benzoate on the prefrontal cortex and hippocampal level of D-alanine as known another substrate with antipsychotic and antidepressant properties and other NMDAR-related amino acids, such as, L-alanine, D-serine, L-serine, D-glutamate, L-glutamate, and glycine levels. Our results indicate that sodium benzoate exerts antipsychotic and antidepressant-like effects without changing the D-serine levels in the brain prefrontal cortex (PFC) and hippocampus. Moreover, D-alanine levels in the PFC and hippocampus did not change. Despite these negative findings regarding the effects of D-amino acids in the PFC and hippocampus, sodium benzoate exhibited antipsychotic and antidepressant-like effects. Thus, the therapeutic effects of sodium benzoate are independent of D-serine or D-alanine levels. In conclusion, sodium benzoate may be effective among patients with schizophrenia or depression; however, the mechanisms of actions remain to be elucidated.

Astrocytic D -amino acid oxidase degrades D -serine in the hindbrain

_D -serine modulates excitatory neurotransmission by binding to N-methyl-_D -aspartate glutamate receptors. _D- amino acid oxidase (DAO) degrades _D -amino acids, such as _D -serine, in the central nervous system, and is associated with neurological and psychiatric disorders. However, cell types that express brain DAO remain controversial, and whether brain DAO influences systemic _D -amino acids in addition to brain _D -serine remains unclear. Here, we created astrocyte-specific DAO-conditional knockout mice. Knockout in glial fibrillary acidic protein (GFAP)-positive cells eliminated DAO expression in the hindbrain and increased _D -serine levels significantly in the cerebellum. Brain DAO did not influence levels of _D -amino acids in the forebrain or periphery. These results show that astrocytic DAO regulates _D -serine specifically in the hindbrain.

Free d-Amino Acids in Salivary Gland in Rat

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.

Enantioselective metabolomics by liquid chromatography-mass spectrometry

Metabolomics strives to capture the entirety of the metabolites in a biological system by comprehensive analysis, often by liquid chromatography hyphenated to mass spectrometry. A particular challenge thereby is the differentiation of structural isomers. Common achiral targeted and untargeted assays do not distinguish between enantiomers. This may lead to information loss. An increasing number of publications demonstrate that the enantiomeric ratio of certain metabolites can be meaningful biomarkers of certain diseases emphasizing the importance of introducing enantioselective analytical procedures in metabolomics. In this work, the state-of-the-art in the field of LC-MS based metabolomics is summarized with focus on developments in the recent decade. Methodologies, tagging strategies, workflows and general concepts are outlined. Selected biological applications in which enantioselective metabolomics has documented its usefulness are briefly discussed. In general, targeted enantioselective metabolomics assays are often based on a direct approach using chiral stationary phases (CSP) with polysaccharide derivatives, macrocyclic antibiotics, chiral crown ethers, chiral ion exchangers, donor-acceptor phases as chiral selectors. Rarely, these targeted assays focus on more than 20 analytes and usually are restricted to a certain metabolite class. In a variety of cases, pre-column derivatization of metabolites has been performed, especially for amino acids, to improve separation and detection sensitivity. Triple quadrupole instruments are the detection methods of first choice in targeted assays. Here, issues like matrix effect, absence of blank matrix impair accuracy of results. In selected applications, multiple heart cutting 2D-LC (RP followed by chiral separation) has been pursued to overcome this problem and alleviate bias due to interferences. Non-targeted assays, on the other hand, are based on indirect approach involving tagging with a chiral derivatizing agent (CDA). Besides classical CDAs numerous innovative reagents and workflows have been proposed and are discussed. Thereby, a critical issue for the accuracy is often neglected, viz. the validation of the enantiomeric impurity in the CDA. The majority of applications focus on amino acids, hydroxy acids, oxidized fatty acids and oxylipins. Some potential clinical applications are highlighted.

D-Serine: A Cross Species Review of Safety

Background:D-Serine, a direct, full agonist at the D-serine/glycine modulatory site of the N-methyl-D-aspartate-type glutamate receptors (NMDAR), has been assessed as a treatment for multiple psychiatric and neurological conditions. Based on studies in rats, concerns of nephrotoxicity have limited D-serine research in humans, particularly using high doses. A review of D-serine's safety is timely and pertinent, as D-serine remains under active study for schizophrenia, both directly (R61 MH116093) and indirectly through D-amino acid oxidase (DAAO) inhibitors. The principal focus is on nephrotoxicity, but safety in other physiologic and pathophysiologic systems are also reviewed. Methods: Using the search terms "D-serine," "D-serine and schizophrenia," "D-serine and safety," "D-serine and nephrotoxicity" in PubMed, we conducted a systematic review on D-serine safety. D-serine physiology, dose-response and efficacy in clinical studies and dAAO inhibitor safety is also discussed. Results: When D-serine doses >500 mg/kg are used in rats, nephrotoxicity, manifesting as an acute tubular necrosis syndrome, seen within hours of administration is highly common, if not universal. In other species, however, D-serine induced nephrotoxicity has not been reported, even in other rodent species such as mice and rabbits. Even in rats, D--serine related toxicity is dose dependent and reversible; and does not appear to be present in rats at doses producing an acute Cmax of <2,000 nmol/mL. For comparison, the Cmax of D-serine 120 mg/kg, the highest dose tested in humans, is ~500 nmol/mL in acute dosing. Across all published human studies, only one subject has been reported to have abnormal renal values related to D-serine treatment. This abnormality did not clearly map on to the acute tubular necrosis syndrome seen in rats, and fully resolved within a few days of stopping treatment. DAAO inhibitors may be nephroprotective. D-Serine may have a physiologic role in metabolic, extra-pyramidal, cardiac and other systems, but no other clinically significant safety concerns are revealed in the literature. Conclusions: Even before considering human to rat differences in renal physiology, using current FDA guided monitoring paradigms, D-serine appears safe at currently studied maximal doses, with potential safety in combination with DAAO inhibitors.

Development of derivatization reagents bearing chiral 4-imidazolidinone for distinguishing primary amines from other amino acids and application to the liquid chromatography-tandem mass spectrometric analysis of miso

We designed and synthesized three novel derivatization reagents bearing chiral 4-imidazolidinone, namely succinimidyl 2-(3-((benzyloxy)carbonyl)-1-methyl, ethyl, and -phenyl-5-oxoimidazolidin-4-yl)acetates (CIMs), for use in liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis. The CIMs were able to discriminate primary amines from other compounds such as secondary amines and phenols, based on their unique m/z reduction of precursor ion to form product ion in MS/MS. As amino acid derivatization reagents, the CIMs were compared in terms of enantioseparation of amino acid and detection sensitivity. CIM_a-OSu with 1-methyl-5-oxoimidazolidinone moiety gave the best optical resolution and detection sensitivity among the CIM reagents. Next, we applied (R)-CIM_a-OSu to determine amino acids in miso by LC-triple-quadrupole MS. The proposed method achieved simultaneous determination of 20 l-amino acids and two d-amino acids (d-alanine and d-serine) in the sample with a high sensitivity (limits of detection 5-238 fmol, signal-to-noise ratio 3.3). After derivatization with CIM_a-OSu, it was possible to determine whether each peak in the chromatogram was a component of primary amine or not, by using a high-resolution orbitrap MS instrument.

Acute D-Serine Co-Agonism of β-Cell NMDA Receptors Potentiates Glucose-Stimulated Insulin Secretion and Excitatory β-Cell Membrane Activity

Insulin-secreting pancreatic β-cells express proteins characteristic of D-serine regulated synapses, but the acute effect of D-serine co-agonism on its presumptive β-cell target, N-methyl D-aspartate receptors (NMDARs), is unclear. We used multiple models to evaluate glucose homeostasis and insulin secretion in mice with a systemic increase in D-serine (intraperitoneal injection or DAAO mutants without D-serine catabolism) or tissue-specific loss of Grin1-encoded GluN1, the D-serine binding NMDAR subunit. We also investigated the effects of D-serine ± NMDA on glucose-stimulated insulin secretion (GSIS) and β-cell depolarizing membrane oscillations, using perforated patch electrophysiology, in β-cell-containing primary isolated mouse islets. In vivo models of elevated D-serine correlated to improved blood glucose and insulin levels. In vitro, D-serine potentiated GSIS and β-cell membrane excitation, dependent on NMDAR activating conditions including GluN1 expression (co-agonist target), simultaneous NMDA (agonist), and elevated glucose (depolarization). Pancreatic GluN1-loss females were glucose intolerant and GSIS was depressed in islets from younger, but not older, βGrin1 KO mice. Thus, D-serine is capable of acute antidiabetic effects in mice and potentiates insulin secretion through excitatory β-cell NMDAR co-agonism but strain-dependent shifts in potency and age/sex-specific Grin1-loss phenotypes suggest that context is critical to the interpretation of data on the role of D-serine and NMDARs in β-cell function.

Total Flavones of Abelmoschus manihot Remodels Gut Microbiota and Inhibits Microinflammation in Chronic Renal Failure Progression by Targeting Autophagy-Mediated Macrophage Polarization

Background

Recently, progression of chronic renal failure (CRF) has been closely associated with gut microbiota dysbiosis and intestinal metabolite-derived microinflammation. In China, total flavones of Abelmoschus manihot (TFA), a component of Abelmoschus manihot, has been widely used to delay CRF progression in clinics for the past two decades. However, the overall therapeutic mechanisms remain obscure. In this study, we designed experiments to investigate the renoprotective effects of TFA in CRF progression and its underlying mechanisms involved in gut microbiota and microinflammation, compared with febuxostat (FEB), a potent non-purine selective inhibitor of xanthine oxidase.

Methods

In vivo, the CRF rat models were induced by uninephrectomy, potassium oxonate, and proinflammatory diet, and received either TFA suspension, FEB, or vehicle after modeling for 28 days. In vitro, the RAW 264.7 cells were exposed to lipopolysaccharide (LPS) with or without TFA or FEB. Changes in parameters related to renal injury, gut microbiota dysbiosis, gut-derived metabolites, and microinflammation were analyzed in vivo. Changes in macrophage polarization and autophagy and its related signaling were analyzed both in vivo and in vitro.

Results

For the modified CRF model rats, the administration of TFA and FEB improved renal injury, including renal dysfunction and renal tubulointerstitial lesions; remodeled gut microbiota dysbiosis, including decreased Bacteroidales and Lactobacillales and increased Erysipelotrichales; regulated gut-derived metabolites, including d-amino acid oxidase, serine racemase, d-serine, and l-serine; inhibited microinflammation, including interleukin 1β (IL1β), tumor necrosis factor-α, and nuclear factor-κB; and modulated macrophage polarization, including markers of M1/M2 macrophages. More importantly, TFA and FEB reversed the expression of beclin1 (BECN1) and phosphorylation of p62 protein and light chain 3 (LC3) conversion in the kidneys by activating the adenosine monophosphate-activated protein kinase-sirtuin 1 (AMPK-SIRT1) signaling. Further, TFA and FEB have similar effects on macrophage polarization and autophagy and its related signaling in vitro.

Conclusion

In this study, we demonstrated that TFA, similar to FEB, exerts its renoprotective effects partially by therapeutically remodeling gut microbiota dysbiosis and inhibiting intestinal metabolite-derived microinflammation. This is achieved by adjusting autophagy-mediated macrophage polarization through AMPK-SIRT1 signaling. These findings provide more accurate information on the role of TFA in delaying CRF progression.

Determination of phenylalanine enantiomers in the plasma and urine of mammals and ᴅ-amino acid oxidase deficient rodents using two-dimensional high-performance liquid chromatography

A two-dimensional (2D) HPLC system focusing on the determination of phenylalanine (Phe) enantiomers in mammalian physiological fluids has been developed. ᴅ-Phe is indicated to have potential values as a disease biomarker and therapeutic molecule in several neuronal and metabolic disorders, thus the regulation of ᴅ-Phe in mammals is a matter of interest. However, the precise determination of amino acid enantiomers is difficult in complex biological samples, and the development of an analytical method with practically acceptable sensitivity, selectivity and throughput is expected. In the present study, a 2D-HPLC system equipped with a reversed-phase column in the 1st dimension and an enantioselective column in the 2nd dimension has been designed, following the fluorescence derivatization of the target amino acid enantiomers with 4-fluoro-7-nitro-2,1,3-benzoxadiazole (NBD-F). The analytical method was validated using both plasma and urine samples, and successfully applied to human, rat and mouse fluids. Trace levels of ᴅ-Phe were determined in the plasma, and the %ᴅ values were around 0.1% for all species. In the urine, relatively large amounts of ᴅ-Phe were observed, and the %ᴅ values for humans, rats and mice were 3.99, 1.76 and 5.25%, respectively. The relationships between the enzymatic activity of ᴅ-amino acid oxidase (DAO) and the amounts of intrinsic ᴅ-Phe have also been clarified, and high ᴅ-Phe amounts were observed (around 0.3% in the plasma and around 50% in the urine) in the DAO deficient rats and mice.

D-Amino acids in mammalian endocrine tissues

D-Aspartate, D-serine and D-alanine are a regular occurrence in mammalian endocrine tissues, though in amounts varying with the type of gland. The pituitary gland, pineal gland, thyroid, adrenal glands and testis contain relatively large amounts of D-aspartate in all species examined. D-alanine is relatively abundant in the pituitary gland and pancreas. High levels of D-serine characterize the hypothalamus. D-leucine, D-proline and D-glutamate are generally low. The current knowledge of physiological roles of D-amino acids in endocrine tissues is far from exhaustive, yet the topic is attracting increasing interest because of its potential in pharmacological application. D-aspartate is known to act at all levels of the hypothalamus-pituitary-testis axis, playing a key role in reproductive biology in several vertebrate classes. An involvement of D-amino acids in the endocrine function of the pancreas is emerging. D-Aspartate has been immunolocalized in insulin-containing secretory granules in INS-1 E clonal β cells and is co-secreted with insulin by exocytosis. Specific immunolocalization of D-alanine in pituitary ACTH-secreting cells and pancreatic β-cells suggests that this amino acid participates in blood glucose regulation in mammals. By modulating insulin secretion, D-serine probably participates in the control of systemic glucose metabolism by modulating insulin secretion. We anticipate that future investigation will significantly increase the functional repertoire of D-amino acids in homeostatic control.

d-Amino acid oxidase deficiency is caused by a large deletion in the Dao gene in LEA rats

d-Amino acids, enantiomers of l-amino acids, are increasingly recognized as physiologically active molecules as well as potential biomarkers for diseases. d-Amino acid oxidase (DAO) catalyzes the oxidative deamination of d-amino acids and is present in a wide variety of organisms from yeasts to humans. Previous studies indicated that LEA rats lacked DAO activity, and levels of d-Ser and d-Ala were markedly increased in their tissues, suggesting a mutated locus responsible for the lack of Dao activity (ldao) existed in the LEA genome. Sequence analysis identified deletion breakpoints located in intron 4-5 of the Dao gene and intron 1-2 of the Svop gene, resulting in a 54.1-kb deletion which encompassed exons 5-12 of the Dao gene and exons 2-16 of the Svop gene. We developed a novel congenic rat strain, F344-Dao^ldao, harboring the Dao^ldao mutation from LEA rats delivered onto the F344 genetic background. Compared to the parental F344 strain, in F344-Dao^ldao rats d-Ala was markedly increased in both cerebrum and cerebellum, while d-Ser content was increased in cerebellum but not cerebrum. d-Ala, d-Ser, d-Pro and d-Leu levels were also elevated in F344-Dao^ldao plasma. F344-Dao^ldao rats represent a novel model system that will aid in elucidating the physiological functions of d-amino acids in vivo. (203 words).

A single administration of 'microbial' D-alanine to healthy volunteers augments reaction to negative emotions: A comparison with D-serine

BACKGROUND

Activation of the glutamate N-methyl-D-aspartate receptor with its co-agonist D-serine has been shown to improve subjective mood in healthy volunteers. D-alanine is another potent N-methyl-D-aspartate receptor co-agonist which arises from the natural breakdown of host gut microbes, and is predominantly sequestered in the pituitary. This may suggest that D-alanine influences the neuroendocrine stress response which may then impact on emotion.

AIMS

The current study explored the effects of D-serine and D-alanine on emotional processing, cognition and the levels of the stress hormone cortisol in healthy volunteers.

METHODS

In a double-blind, placebo-controlled randomised study, participants (n=63) received a single oral dose of either D-serine, D-alanine (60 mg/kg) or placebo and then performed the Emotional Test Battery and N-back task (two hours post-administration) and provided saliva samples at fixed intervals.

RESULTS

Subjects administered with D-alanine were faster at identifying facial expressions of fear, surprise and anger, and at categorising negative self-referential words. Participants on D-alanine also showed a trend to recall more words than placebo in a memory task. D-serine did not have any meaningful effects in any of the tasks. Neither amino acid had a significant effect on salivary cortisol or working memory.

CONCLUSION

This study is the first to suggest that D-alanine can modulate emotional cognitive processing after a single dose. The lack of findings for D-serine nevertheless contrasts a previous study, emphasising a need for further investigation to clarify discrepancies. A better understanding of the physiological actions of D-amino acids would be beneficial in evaluating their therapeutic potential.

The glycine site of NMDA receptors: A target for cognitive enhancement in psychiatric disorders

Cognitive dysfunction is a principal determinant of functional impairment in major depressive disorder (MDD) and often persists during periods of euthymia. Abnormalities in the glutamate system, particularly in N-methyl-d-aspartate receptors (NMDARs) activity, have been shown to contribute to both mood and cognitive symptoms in MDD. The current narrative review aims to evaluate the potential pro-cognitive effects of targeting the glycine site of NMDARs in the treatment of psychiatric disorders, with a special focus on how these results may apply to MDD. Literature databases were searched from inception to May 2018 for relevant pre-clinical and clinical studies evaluating antidepressant and pro-cognitive effects of NMDAR glycine site modulators in both MDD and non-MDD samples. Six glycine site modulators with pro-cognitive and antidepressant properties were identified: d-serine (co-agonist), d-cycloserine (partial agonist), d-alanine (co-agonist), glycine (agonist), sarcosine (co-agonist) and rapastinel (partial agonist). Preclinical animal studies demonstrated improved neuroplasticity and pro-cognitive effects with these agents. Numerous proof-of-concept clinical trials demonstrated pro-cognitive and antidepressant effects trans-diagnostically (e.g., in healthy participants, MDD, schizophrenia, anxiety disorders, major neurocognitive disorders). The generalizability of these clinical studies was limited by the small sample sizes and the paucity of studies directly evaluating cognitive effects in MDD samples, as most clinical trials were in non-MDD samples. Taken together, preliminary results suggest that the glycine site of NMDARs is a promising target to ameliorate symptoms of depression and cognitive dysfunction. Additional rigorously designed clinical studies are required to determine the cognitive effects of these agents in MDD.

Investigating brain d-serine: Advocacy for good practices

The last two decades have witnessed remarkable advance in our understanding the role of d-amino acids in the mammalian nervous system: from the unknown, to known molecules with unknown functions, to potential central players in health and disease. d-Amino acids have emerged as an important class of signaling molecules. In particular, the exploration of the roles of d-serine in brain physiopathology is a vibrant field that is growing at an accelerating pace. However, disentangling the functions of a chiral molecule in a complex chemical matrice as the brain requires specific measurement and detection methods but is also a challenging task as many molecular tools and models investigators are using can lead to confounded observations. Thus, study of d-amino acids demands accurate methodologies and specific controls, and these have often been lacking. Here we outline best practices for d-amino acid research, with a special emphasis on d-serine. We hope these concepts help move the field to greater rigor and reproducibility, allowing the field to advance.

Lack of Effect of Sodium Benzoate at Reported Clinical Therapeutic Concentration on d-Alanine Metabolism in Dogs

Cognitive decline and psychosis have been hypothesized to be mediated by N-methyl-d-aspartate receptor (NMDAR) hypofunction. Consistent with this hypothesis, chronic treatment with d-alanine, a coagonist at the glycine site of the NMDAR, leads to an improvement of positive and cognitive symptoms in schizophrenic patients. d-alanine is oxidized by d-amino acid oxidase (DAAO); thus, an inhibitor of DAAO would be expected to enhance d-alanine levels and likewise lead to desirable clinical outcomes. Sodium benzoate, on the basis of d-amino acid inhibition, was observed to display beneficial clinical effects in schizophrenic and Alzheimer's patients. However, in the clinical pilot studies using sodium benzoate, d-amino acids were not quantified to verify that sodium benzoate's efficacy was mediated through DAAO inhibition. In this study, d-alanine content was monitored in cerebral spinal fluid (CSF) of dogs treated with daily injections of d-alanine (30 mg/kg) alone and in combination with sodium benzoate (30 mg/kg) for seven consecutive days. We reasoned that the cerebral spinal fluid d-alanine quantity is reflective of the brain d-alanine levels and it would increase as a consequence of DAAO inhibition with sodium benzoate. We found that d-alanine treatment lead to maximal concentration of 7.51 μM CSF d-alanine level; however, coadministration of sodium benzoate and d-alanine did not change CSF d-alanine level beyond that of d-alanine treatment alone. As a consequence, we conclude that clinical efficacy associated with chronic administration of sodium benzoate in schizophrenic and Alzheimer's patients is likely not mediated through inhibition of DAAO.

Gut microbiota-derived D-serine protects against acute kidney injury

Gut microbiota-derived metabolites play important roles in health and disease. D-amino acids and their L-forms are metabolites of gut microbiota with distinct functions. In this study, we show the pathophysiologic role of D-amino acids in association with gut microbiota in humans and mice with acute kidney injury (AKI). In a mouse kidney ischemia/reperfusion model, the gut microbiota protected against tubular injury. AKI-induced gut dysbiosis contributed to the altered metabolism of D-amino acids. Among the D-amino acids, only D-serine was detectable in the kidney. In injured kidneys, the activity of D-amino acid oxidase was decreased. Conversely, the activity of serine racemase was increased. The oral administration of D-serine mitigated the kidney injury in B6 mice and D-serine-depleted mice. D-serine suppressed hypoxia-induced tubular damage and promoted posthypoxic tubular cell proliferation. Finally, the D-serine levels in circulation were significantly correlated with the decrease in kidney function in AKI patients. These results demonstrate the renoprotective effects of gut-derived D-serine in AKI, shed light on the interactions between the gut microbiota and the kidney in both health and AKI, and highlight D-serine as a potential new therapeutic target and biomarker for AKI.

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.

Emerging Role of D-Amino Acid Metabolism in the Innate Defense

Mammalian innate and adaptive immune systems use the pattern recognition receptors, such as toll-like receptors, to detect conserved bacterial and viral components. Bacteria synthesize diverse D-amino acids while eukaryotes and archaea generally produce two D-amino acids, raising the possibility that many of bacterial D-amino acids are bacteria-specific metabolites. Although D-amino acids have not been identified to bind to any known pattern recognition receptors, D-amino acids are enantioselectively recognized by some other receptors and enzymes including a flavoenzyme D-amino acid oxidase (DAO) in mammals. At host-microbe interfaces in the neutrophils and intestinal mucosa, DAO catalyzes oxidation of bacterial D-amino acids, such as D-alanine, and generates H₂O₂, which is linked to antimicrobial activity. Intestinal DAO also modifies the composition of microbiota through modulation of growth for some bacteria that are dependent on host nutrition. Furthermore, regulation and recognition of D-amino acids in mammals have additional meanings at various host-microbe interfaces; D-phenylalanine and D-tryptophan regulate chemotaxis of neutrophils through a G-coupled protein receptor, D-serine has a bacteriostatic role in the urinary tract, D-phenylalanine and D-leucine inhibit innate immunity through the sweet taste receptor in the upper airway, and D-tryptophan modulates immune tolerance in the lower airway. This mini-review highlights recent evidence supporting the hypothesis that D-amino acids are utilized as inter-kingdom communication at host-microbe interface to modulate bacterial colonization and host defense.

Heterogeneity of D-Serine Distribution in the Human Central Nervous System

D-serine is an endogenous ligand for N-methyl-D-aspartate glutamate receptors. Accumulating evidence including genetic associations of D-serine metabolism with neurological or psychiatric diseases suggest that D-serine is crucial in human neurophysiology. However, distribution and regulation of D-serine in humans are not well understood. Here, we found that D-serine is heterogeneously distributed in the human central nervous system (CNS). The cerebrum contains the highest level of D-serine among the areas in the CNS. There is heterogeneity in its distribution in the cerebrum and even within the cerebral neocortex. The neocortical heterogeneity is associated with Brodmann or functional areas but is unrelated to basic patterns of cortical layer structure or regional expressional variation of metabolic enzymes for D-serine. Such D-serine distribution may reflect functional diversity of glutamatergic neurons in the human CNS, which may serve as a basis for clinical and pharmacological studies on D-serine modulation.

Intraperitoneal injection of d-serine inhibits high-fat diet intake and preference in male mice

d-serine is a co-agonist of the N-methyl d-aspartate (NMDA) receptor, an important modulator of glutamatergic excitatory synaptic transmission. We previously reported that oral d-serine ingestion inhibited the intake of highly preferred food and promoted the intake of less preferred food in mice. Here, we analyzed the effects of intraperitoneal (IP) d-serine injections on feeding behavior in mice. We assessed the effects of d-serine during both the acquisition and maintenance of a preference for high-fat diets (HFDs). Aversiveness of IP d-serine was analyzed in the conditioned taste aversion paradigm. The effects on food intake were assessed by providing liquid meals with different fat contents. Finally, we measured brain d-serine and l-serine levels after d-serine administration. We found that IP-injected d-serine effectively inhibited the acquisition of a HFD preference, but failed to prevent expression of a previously learned HFD preference. IP-injected d-serine was not sufficient to condition taste aversion. The effect on HFD preference acquisition was associated with increases in d-serine levels in the cerebral cortex, hypothalamus, and cerebellum. IP-injected d-serine most effectively inhibited the intake of liquid meals with high fat content. This effect was dose-dependent, but the responses varied significantly among male C57BL/6J mice. The differential responses to d-serine were consistent among multiple trials in each mouse. In summary, IP-injected d-serine inhibited HFD intake and the acquisition of an HFD preference. Individual mice with the same genetic background showed different sensitivities to d-serine; thus, d-serine sensitivity may be associated with unidentified traits.

Serine racemase is expressed in islets and contributes to the regulation of glucose homeostasis

NMDA receptors (NMDARs) have recently been discovered as functional regulators of pancreatic β-cell insulin secretion. While these excitatory receptor channels have been extensively studied in the brain for their role in synaptic plasticity and development, little is known about how they work in β-cells. In neuronal cells, NMDAR activation requires the simultaneous binding of glutamate and a rate-limiting co-agonist, such as D-serine. D-serine levels and availability in most of the brain rely on endogenous synthesis by the enzyme serine racemase (Srr). Srr transcripts have been reported in human and mouse islets but it is not clear whether Srr is functionally expressed in β-cells or what its role in the pancreas might be. In this investigation, we reveal that Srr protein is highly expressed in primary human and mouse β-cells. Mice with whole body deletion of Srr (Srr KO) show improved glucose tolerance through enhanced insulin secretory capacity, possibly through Srr-mediated alterations in islet NMDAR expression and function. We observed elevated insulin sensitivity in some animals, suggesting Srr metabolic regulation in other peripheral organs as well. Srr expression in neonatal and embryonic islets, and adult deficits in Srr KO pancreas weight and islet insulin content, point toward a potential role for Srr in pancreatic development. These data reveal the first evidence that Srr may regulate glucose homeostasis in peripheral tissues and provide circumstantial evidence that D-serine may be an endogenous islet NMDAR co-agonist in β-cells.

Oral administration of D-alanine in monkeys robustly increases plasma and cerebrospinal fluid levels but experimental D-amino acid oxidase inhibitors had minimal effect

Hypofunction of the N-methyl-d-aspartate (NMDA) receptor is thought to exacerbate psychosis in patients diagnosed with schizophrenia. Consistent with this hypothesis, D-alanine, a co-agonist at the glycine site of the NMDA receptor, was shown to improve positive and cognitive symptoms when used as add-on therapy for schizophrenia treatment. However, D-alanine had to be administered at high doses (~7 g) to observe clinical effects. One possible reason for the high dose is that D-alanine could be undergoing oxidation by D-amino acid oxidase (DAAO) before it reaches the brain. If this is the case, the dose could be reduced by co-administration of D-alanine with a DAAO inhibitor (DAAOi). Early studies with rodents showed that co-administration of D-alanine with 5-chloro-benzo[d]isoxazol-3-ol (CBIO), a prototype DAAOi, significantly enhanced the levels of extracellular D-alanine in the frontal cortex compared with D-alanine alone. Further, the use of CBIO reduced the dose of D-alanine needed to attenuate prepulse inhibition deficits induced by dizocilpine. The objective of the work reported herein was to confirm the hypothesis that DAAO inhibition can enhance D-alanine exposure in a species closer to humans: non-human primates. We report that while oral D-alanine administration to baboons (10 mg/kg) enhanced D-alanine plasma and CSF levels over 20-fold versus endogenous levels, addition of experimental DAAOi to the regimen exhibited a 2.2-fold enhancement in plasma and no measurable effect on CSF levels. The results provide caution regarding the utility of DAAO inhibition to increase D-amino acid levels as treatment for patients with schizophrenia.

Enantioselective analysis of proteinogenic amino acids in cerebrospinal fluid by capillary electrophoresis-mass spectrometry

d-Amino acids (AAs) are increasingly being recognized as essential molecules in biological systems. Enantioselective analysis of proteinogenic AAs in biological samples was accomplished by CE-MS employing β-CD as chiral selector and ESI via sheath-liquid (SL) interfacing. Prior to analysis, AAs were fully derivatized with FMOC, improving AA-enantiomer separation and ESI efficiency. In order to optimize the separation and MS detection of FMOC-AAs, the effects of type and concentration of CD in the BGE, the composition of the SL, and MS-interfacing parameters were evaluated. Using a BGE of 10 mM β-CD in 50 mM ammonium bicarbonate (pH 8) containing 15% v/v isopropanol, a SL of isopropanol-water-1 M ammonium bicarbonate (50:50:1, v/v/v) at a flow rate of 3 μL/min, and a nebulizer gas pressure of 2 psi, 15 proteinogenic AAs could be detected with enantioresolutions up to 3.5 and detection limits down to 0.9 μM (equivalent to less than 3 pg AA injected). The selectivity of the method was demonstrated by the analysis of spiked cerebrospinal fluid, allowing specific detection of d-AAs. Repeatability and linearity obtained for cerebrospinal fluid were similar to standard solutions, with peak area and migration-time RSDs (n = 5) below 16.2 and 1.6%, respectively, and a linear response (R(2) ≥ 0.977) in the 3-90 μM range.

Increased sensitivity to inflammatory pain induced by subcutaneous formalin injection in serine racemase knock-out mice

D-Serine, an endogenous coagonist of the N-methyl-D-aspartate receptor (NMDAR), is widely distributed in the central nervous system and is synthesized from L-serine by serine racemase (SR). NMDAR plays an important role in pain processing including central sensitization that eventually causes hyperalgesia. To elucidate the roles of D-serine and SR in pain transmission, we evaluated the behavioral changes and spinal nociceptive processing induced by formalin using SR knock-out (KO) mice. We found that SR is mainly distributed in lamina II of the dorsal horn of the spinal cord in wild-type (WT) mice. Although the formalin injected subcutaneously induced the biphasic pain response of licking in SR-KO and WT mice, the time spent on licking was significantly longer in the SR-KO mice during the second phase of the formalin test. The number of neurons immunopositive for c-Fos and phosphorylated extracellular signal-regulated kinase (p-ERK), which are molecular pain markers, in laminae I-II of the ipsilateral dorsal horn was significantly larger in the SR-KO mice. Immunohistochemical staining revealed that the distribution of SR changed from being broad to being concentrated in cell bodies after the formalin injection. On the other hand, the expression level of the cytosolic SR in the ipsilateral dorsal horn significantly decreased. Oral administration of 10 mM D-serine in drinking water for one week cancelled the difference in pain behaviors between WT and SR-KO mice in phase 2 of the formalin test. These findings demonstrate that the SR-KO mice showed increased sensitivity to inflammatory pain and the WT mice showed translocation of SR and decreased SR expression levels after the formalin injection, which suggest a novel antinociceptive mechanism via SR indicating an important role of D-serine in pain transmission.