d-Amino acids: new clinical pathways for brain diseases

Early involvement of D-serine in β-amyloid-dependent pathophysiology

The N-methyl-D-aspartate subtype of glutamate receptors (NMDAR) is a key regulator of brain plasticity encoding learning and memory. In addition to glutamate, NMDAR activation requires the binding of the co-agonist D-serine. The beta-amyloid (Aß) peptide which accumulates in Alzheimer's disease (AD), affects the D-serine-dependent NMDAR activation in vitro, but whether this alteration would significantly contribute to AD-related pathophysiology and memory deficits remains unclear. Herein, we report a decrease in the maximal pool of recruitable NMDAR and in the expression of NMDAR-dependent long-term potentiation together with impaired basal neurotransmission at CA3/CA1 synapses from hippocampal slices of 5xFAD mouse, an AD-related model with elevated Aß levels. The NMDAR synaptic impairments develop from 1.5 to 2 months of age with the initial rise of Aß and is correlated to a transient increase in D-serine levels. Deficits in working and spatial memories as well as cognitive flexibility then occurred in 10-12 months-old animals. Importantly, the NMDA-related synaptic deregulations (but not the altered basal neurotransmission) and behavioral impairments (working and cognitive flexibility) are prevented or reduced (spatial memory) in 5xFAD mice devoid of D-serine after genetic deletion of its synthesis enzyme serine racemase. Altogether, these results therefore provide in vivo evidence for the implication of D-serine at least in the early pathogenic signatures of AD driven by the increase in amyloid load suggesting that the recent proposal of preventive therapy of AD by administration of the precursor L-serine remains questionable.

Chiral Optical Sensing of Amino Acids with 2-Trifluoromethyl Benzaldehyde for Ophiopogon japonicus Authentication

The detection and analysis of chiral molecules have long been challenging in analytical chemistry. This study introduces a novel approach that utilizes 2-trifluoromethyl benzaldehyde as a small-molecule probe capable of forming a stable Schiff base with chiral amino acids in aqueous solution under alkaline conditions. The amino acid Schiff bases present a strong Cotton effect and UV absorption at wavelengths exceeding 260 nm, enabling chiral analysis, including assignment of absolute configuration, enantiomeric composition, and total concentration. An application of this method was the authentication of the herbal medicine Ophiopogon japonicus. Using principal component analysis and orthogonal partial least squares discriminant analysis, we successfully differentiated O. japonicus samples collected in two distinct locations with 20 samples. This rapid and convenient method offers a new approach to quality control of herbal medicine.

Examination of the relationship between D-amino acid profiles and cognitive function in individuals with mild cognitive impairment: a machine learning approach

BACKGROUND

The global prevalence of dementia is significantly increasing. Early detection and prevention strategies, particularly for mild cognitive impairment (MCI), are crucial but currently hindered by the lack of established biomarkers. Here, we aimed to develop a high-precision screening method for MCI by combining D-amino acid profiles from peripheral blood samples with noninvasive subject information using nonlinear machine learning (ML) algorithms.

METHODS

A cross-sectional study was conducted with 200 participants aged 50-89 years, classified into cognitively normal and MCI-suspected groups based on Mini-Mental State Examination scores. High-throughput techniques were used to analyze the D-amino acid profiles, specifically D-alanine (%) and D-proline (%), in peripheral blood. Correlation analysis was performed between D-amino acid levels in venous and fingertip blood. The predictive performance of various ML models, including Logistic Regression, Random Forest, kernel Support Vector Machine (SVM), and Artificial Neural Network (ANN), was compared.

RESULTS

Nonlinear models (kernel SVM and ANN) that combined D-amino acid profiles with subject information achieved the highest area under the curve values of 0.78 and 0.79, respectively, demonstrating that the combination of D-amino acid profiles and noninvasive subject information is effective in detecting MCI.

CONCLUSIONS

Combining D-amino acid profiles with noninvasive subject information using nonlinear ML models, particularly kernel SVM and ANN, shows promise as a high-precision screening tool for MCI. This approach could serve as a cost-effective preliminary screening method before more invasive and expensive diagnostic tests and significantly contribute to the early detection and development of intervention strategies for dementia.

Chirality Quantification for High-Performance Nanophotonic Biosensors

Recent advancements in chiral metabolomics have facilitated the discovery of disease biomarkers through the enantioselective measurement of metabolites, offering new opportunities for diagnosis, prognosis, and personalized medicine. Although chiral photonic nanomaterials have emerged as promising platforms for chiral biosensing, enhancing sensitivity and enabling the detection of biomolecules at extremely low concentrations, a deeper understanding of the relationship between structural and optical chirality is crucial for optimizing these platforms. This perspective examines recent methods for quantifying chirality, including the Hausdorff Chirality Measure (HCM), Continuous Chirality Measure (CCM), Osipov-Pickup-Dunmur (OPD), and Graph-Theoretical Chirality (GTC) measure. These approaches have advanced the understanding of chirality in both materials and biomolecules, as well as its correlation with optical responses. This work emphasizes the role of chiral quantification in improving biosensor performance and explores the potential of near-field chiroptical studies to enhance sensor capabilities. Finally, this work addresses key challenges and outline future research directions for advancing chiral biosensors, with a focus on improving nano-bio interface interactions to drive the development of next-generation sensing technologies.

A bifunctional coumarin-based CD probe for chiral analysis of amino acids in aqueous solution

Amino acids play important roles in human pathology and physiology and the qualitative and quantitative determination of chiral amino acids in humans and mammals also has important impacts on the life sciences. Therefore, the introduction of artificial probes to assess the concentrations and enantiomeric compositions [ee = ([D] - [L])/([D] + [L])] of amino acids in aqueous solution is necessary in understanding certain biological processes and diagnosing and treating diseases. Herein, a bifunctional achiral coumarin probe (Br-Coumarin) is reported to determine the absolute configuration, ee value, and concentration of 16 amino acids in THF/H2O = 1/4 solution at micromolar concentrations. The effectiveness and practicability of the sensing methods are illustrated through the evaluation of various ee values and concentrations of Cys, Pro, and Phe samples with good accuracy. Besides, the reactions of the probe with various amino acids exhibit different colors under daylight or UV light (365 nm). Particularly, Br-Coumarin can be used to visually detect Cys, Lys, Arg, and Pro with high selectivity under both daylight and UV light (365 nm) and exhibits excellent selectivity and sensitivity for the fluorescent recognition of Cys.

Copper in human health: From COVID 19 to neurodegenerative diseases

Copper (Cu) exists in two oxidation states Cu+I and Cu+II yielding formation of enzymes involved in biological processes. In higher concentrations, by oxidative process and ROS production, Cu is toxic towards plants, humans and animals livers as observed in Wilson disease or sheep scrapie. Fighting according to the Fenton reaction against bacteria and viruses, has been proposed as a mean of combatting nosocomial diseases and complementary to COVID19 vaccination. In humans, Cu is stocked in liver, muscle or bound to brain protein as ß-APP, tau-protein, α-synuclein, ubiquitin or prion which present antioxidant properties when Cu-bonded. In abnormal ß-sheet conformation, they can trigger neurodegenerative diseases such as Alzheimer(AD), Parkinson(PD) and ALS. In these diseases, blood copper increase correlated with brain copper decrease has been described. In AD, abnormal D-serine has been detected in blood and cerebrospinal fluid. D-glutamate and D-alanine blood levels have been found in AD and could also be controlled with Cu and ceruloplasmin in a possible disease screening test. This abnormal D-conformation might result from epimerization of physiologically L-conformation brain peptides into protease-resistant D-enantiomers. This has previously been experimentally demonstrated for Bovine Spongiform Encephalopathy in a free Cu reductive medium with UV-induced free radicals. The Cu brain protective effect against free radicals was restored with cupric addition in oxidizing medium. Cupric supplementation in the brain, might restore Cu protection and slow down neurodegenerative processes. To lower side effects, Cu amino-acid complexes able to cross the blood brain barrier might be suggested for a Cu transfer to the brain.

Efficient gene delivery admitted by small metabolites specifically targeting astrocytes in the mouse brain

The development of efficient and targeted methods for delivering DNA in vivo has long been a major focus of research. In this study, we introduce a gene delivery approach admitted by small metabolites (gDAM) for the efficient and targeted delivery of naked DNA into astrocytes in the adult brains of mice. gDAM uses a straightforward combination of DNA and small metabolites, including glycine, L-proline, L-serine, L-histidine, D-alanine, Gly-Gly, and Gly-Gly-Gly, to achieve astrocyte-specific delivery of naked DNA, resulting in transient and robust gene expression in these cells. Using gDAM, we successfully co-deliver the PiggyBac transposon and the CRISPR-Cas9 system to induce long-term overexpression of the oncogene EGFRvIII and knockout of tumor suppressor genes Nf1, Pten, and Trp53 in astrocytes, leading to the development of astrocyte-derived gliomas in immunocompetent mice. Furthermore, gDAM facilitates the delivery of naked DNA to peripheral glioma astrocytes. The overexpression of interferon-β and granulocyte-macrophage colony-stimulating factor in these peripheral glioma astrocytes significantly prolongs the overall survival of mice bearing 73C glioma cells. This approach offers a new perspective on developing gene delivery systems that specifically target astrocytes to meet the varied needs of both research and gene therapy. The innovative strategy behind gDAM is expected to provide fresh inspiration in the quest for DNA delivery to other tissues, such as skeletal muscle and skin.

Excitatory amino acids as therapeutic agents: Reversing neurodegenerative trajectory by tackling excitotoxicity

Neurodegenerative diseases pose significant challenges to healthcare systems globally due to their complex etiology and relentless progression, often rendering conventional treatments ineffective. Recent advances have spotlighted excitatory amino acids, particularly D-amino acids, once considered as products of metabolism of the microbiota or deriving from food intake. This review explores the role of D-amino acids in mitigating excitotoxicity-a process characterized by excessive calcium influx through aberrant N-methyl-D-aspartate receptor (NMDAR) activation, which is implicated in the pathogenesis of diseases like Alzheimer's disease. By providing alternative pathways for neuronal signaling and protecting against excitotoxic damage, D-amino acids offer a novel approach to reversing neurodegenerative trajectories. Future research should focus on elucidating the detailed mechanisms of action of these compounds, evaluating their therapeutic potential through rigorous preclinical and clinical trials, and developing effective delivery systems to optimize their neuroprotective effects. This emerging field holds promise for developing innovative treatment strategies that could significantly improve outcomes for patients with neurodegenerative disorders.

The Potential Mechanism of D-Amino Acids - Mitochondria Axis in the Progression of Diabetic Kidney Disease

Diabetic kidney disease (DKD) is a major complication of diabetes mellitus (DM) and stands out as the leading cause of end-stage renal disease worldwide. There is increasing evidence that mitochondrial dysfunction, including impaired mitochondrial biogenesis, dynamics, and oxidative stress, contributes to the development and progression of DKD. D-amino acids (D-AAs), which are enantiomers of L-AAs, have recently been detected in various living organisms and are acknowledged to play important roles in numerous physiological processes in the human body. Accumulating evidence demonstrates that D-AA levels in blood or urine could serve as useful biomarkers for reflecting renal function. The physiological roles of D-AAs are implicated in the regulation of cellular proliferation, oxidative stress, generation of reactive oxygen species (ROS), and innate immunity. This article reviews current evidence relating to D-AAs and mitochondrial dysfunction and proposes a potential interaction and contribution of the D-AAs-mitochondria axis in DKD pathophysiology and progression. This insight could provide novel therapeutic approaches for preventing or ameliorating DKD based on this biological axis.

Age- and Sex-Specific Regulation of Serine Racemase in the Retina of an Alzheimer's Disease Mouse

Purpose

Changes associated with Alzheimer's disease (AD) may have measurable effects on the retina, which may facilitate early detection due to the eye's accessibility. Retinal pathology and the regulation of serine racemase (SR) were investigated in the retinas of APP(SW)/PS1(∆E9) mice.

Methods

SR in the retinas and the content of D-serine in the aqueous humor were analyzed. The structure and function of the retina were assessed. Additionally, the regulation of SR in primary Müller cell cultures was investigated.

Results

SR levels were significantly higher in the retinas of 18- and 24-month-old male APP/PS1 mice, whereas aqueous humor D-serine was lower in 24-month-old APP/PS1 male mice compared to wild-type (WT) mice. Neither Aβ nor 17β-estradiol increased SR, but the combination of both did in Müller cell cultures. In contrast, 17β-estradiol increased Srr mRNA in the cultures. At 8 months of age, male APP/PS1 mice began to display reduced b-wave amplitude in scotopic and photopic electroretinography (ERG) recordings, unlike female APP/PS1 mice. Although the retinal layer thickness in APP/PS1 mice did not differ from WT mice, there was overt apoptosis in the inner and outer nuclear layers of the APP/PS1 mice retinas.

Conclusions

The age- and sex-specific regulation of SR is correlated with the pathology of an AD retina. Because the time window for SR regulation and D-serine alteration occurs after photoreceptor dysfunction in the AD retinas, it has limited value as a detection biomarker but may be useful as a topographic biomarker for staging severity and monitoring drug interventions in the eye or central nervous system.

Advances in Chiral Metabolomic Profiling and Biomarker Discovery

Chiral metabolomics entails the enantioselective measurement of the metabolome present in a biological system. Over recent years, it has garnered significant interest for its potential in discovering disease biomarkers and aiding clinical diagnostics. D-Amino acids and D-hydroxy acids, traditionally overlooked as unnatural, are now emerging as novel signaling molecules and potential biomarkers for a range of metabolic disorders, brain diseases, kidney disease, diabetes, and cancer. Despite their significance, simultaneous measurements of multiple classes of chiral metabolites in a biological system remain challenging. Hence, limited information is available regarding the metabolic pathways responsible for synthesizing D-amino/hydroxy acid and their associated pathophysiological mechanisms in various diseases. Capitalizing on recent advancements in sensitive analytical techniques, researchers have developed various targeted chiral metabolomic methods for the analysis of chiral biomarkers. Here, we highlight the pivotal role of chiral metabolic profiling studies in disease diagnosis, prognosis, and therapeutic interventions. Furthermore, we describe cutting-edge chromatographic and mass spectrometry methods that enable enantioselective analysis of chiral metabolites. These advanced techniques are instrumental in unraveling the complexities of disease biomarkers, contributing to the ongoing efforts in disease biomarker discovery.

Variations of blood D-serine and D-aspartate homeostasis track psychosis stages

Schizophrenia (SCZ) is a severe psychotic disorder characterized by a disruption in glutamatergic NMDA receptor (NMDAR)-mediated neurotransmission. Compelling evidence has revealed that NMDAR activation is not limited to L-glutamate, L-aspartate, and glycine since other free amino acids (AAs) in the atypical D-configuration, such as D-aspartate and D-serine, also modulate this class of glutamatergic receptors. Although dysregulation of AAs modulating NMDARs has been previously reported in SCZ, it remains unclear whether distinct variations of these biomolecules occur during illness progression from at-risk premorbid to clinically manifest stage. To probe this issue, we used High-Performance Liquid Chromatography (HPLC) to measure serum levels of D- and L-AAs that stimulate NMDARs across four groups of individuals diagnosed with (a) At-Risk Mental State (ARMS) for psychosis, (b) First Episode of Psychosis (FEP), (c) full-blown SCZ and (d) Healthy Donors (HD). We examined how diagnosis, demographic features, and antipsychotic treatment influence the variation of AA levels throughout psychosis progression. Finally, we explored the potential association between AA blood concentrations and clinical and cognitive measures related to psychosis. Our findings identified inter-group differences in serum AA composition, highlighting that the upregulation of D-serine/total serine and D-aspartate/total aspartate ratios represent a peculiar blood biochemical signature of early stages of psychosis progression, while increased L-glutamate, L-aspartate and glycine associate with chronic SCZ diagnosis. The present findings provide direct evidence for early dysregulation of D-AA metabolism and have potential implications for the identification of biomarkers for the early detection and staging of psychosis.

Targeted Chiral Metabolomics of D-Amino Acids: Their Emerging Role as Potential Biomarkers in Neurological Diseases with a Focus on Their Liquid Chromatography-Mass Spectrometry Analysis upon Chiral Derivatization

In neuroscience research, chiral metabolomics is an emerging field, in which D-amino acids play an important role as potential biomarkers for neurological diseases. The targeted chiral analysis of the brain metabolome, employing liquid chromatography (LC) coupled to mass spectrometry (MS), is a pivotal approach for the identification of biomarkers for neurological diseases. This review provides an overview of D-amino acids in neurological diseases and of the state-of-the-art strategies for the enantioselective analysis of chiral amino acids (AAs) in biological samples to investigate their putative role as biomarkers for neurological diseases. Fluctuations in D-amino acids (D-AAs) levels can be related to the pathology of neurological diseases, for example, through their role in the modulation of N-methyl-D-aspartate receptors and neurotransmission. Because of the trace presence of these biomolecules in mammals and the complex nature of biological matrices, highly sensitive and selective analytical methods are essential. Derivatization strategies with chiral reagents are highlighted as critical tools for enhancing detection capabilities. The latest advances in chiral derivatization reactions, coupled to LC-MS/MS analysis, have improved the enantioselective quantification of these AAs and allow the separation of several chiral metabolites in a single analytical run. The enhanced performances of these methods can provide an accurate correlation between specific D-AA profiles and disease states, allowing for a better understanding of neurological diseases and drug effects on the brain.

Mammalian D-Cysteine controls insulin secretion in the pancreas

BACKGROUND

D-amino acids are being recognized as important molecules in mammals with function. This is a first identification of endogenous D-cysteine in mammalian pancreas.

METHODS

Using a novel stereospecific bioluminescent assay, chiral chromatography, enzyme kinetics and a transgenic mouse model we identify endogenous D-cysteine. We elucidate its function in two mice models of type 1 diabetes (STZ and NOD), and in tests of Glucose Stimulated Insulin Secretion in isolated mouse and human islets and INS-1 832/13 cell line.

RESULTS AND DISCUSSION

D-cysteine is synthesized by serine racemase (SR) and SR-/- mice produce 6-10 fold higher levels of insulin in the pancreas and plasma including higher glycogen and ketone bodies in the liver. The excess insulin is stored as amyloid in secretory vesicles and exosomes. In glucose stimulated insulin secretion in mouse and human islets, equimolar amount of D-cysteine showed higher inhibition of insulin secretion compared to D-serine, another closely related stereoisomer synthesized by SR. In mouse models of diabetes (Streptozotocin (STZ) and Non Obese Diabetes (NOD) and human pancreas, the diabetic state showed increased expression of D-cysteine compared to D-serine followed by increased expression of SR. SR-/- mice show decreased cAMP in the pancreas, lower DNA methyltransferase enzymatic and promoter activities followed by reduced phosphorylation of CREB (S133), resulting in decreased methylation of the Ins1 promoter. D-cysteine is efficiently metabolized by D-amino acid oxidase and transported by ASCT2 and Asc1. Dietary supplementation with methyl donors restored the high insulin levels and low DNMT enzymatic activity in SR-/- mice.

CONCLUSIONS

Our data show that endogenous D-cysteine in the mammalian pancreas is a regulator of insulin secretion.

The cell-permeant antioxidant D-thiol ester D-cysteine ethyl ester overcomes physical dependence to morphine in male Sprague Dawley rats

The ability of morphine to decrease cysteine transport into neurons by inhibition of excitatory amino acid transporter 3 (EAA3) may be a key molecular mechanism underlying the acquisition of physical and psychological dependence to morphine. This study examined whether co-administration of the cell-penetrant antioxidant D-thiol ester, D-cysteine ethyl ester (D-CYSee), with morphine, would diminish the development of physical dependence to morphine in male Sprague Dawley rats. Systemic administration of the opioid receptor antagonist, naloxone (NLX), elicited pronounced withdrawal signs (e.g., wet-dog shakes, jumps, rears, circling) in rats that received a subcutaneous depot of morphine (150 mg/kg, SC) for 36 h and continuous intravenous infusion of vehicle (20 μL/h, IV). The NLX-precipitated withdrawal signs were reduced in rats that received an infusion of D-CYSee, but not D-cysteine, (both at 20.8 μmol/kg/h, IV) for the full 36 h. NLX elicited pronounced withdrawal signs in rats treated for 48 h with morphine (150 mg/kg, SC), plus continuous infusion of vehicle (20 μL/h, IV) that began at the 36 h timepoint of morphine treatment. The NLX-precipitated withdrawal signs were reduced in rats that received a 12 h infusion of D-CYSee, but not D-cysteine, (both at 20.8 μmol/kg/h, IV) that began at the 36 h timepoint of morphine treatment. These findings suggest that D-CYSee may attenuate the development of physical dependence to morphine and reverse established dependence to the opioid in male Sprague Dawley rats. Alternatively, D-CYSee may simply suppress the processes responsible for NLX-precipitated withdrawal. Nonetheless, D-CYSee and analogues may be novel therapeutics for the treatment of opioid use disorders.

Emerging Perspectives on Neuroprotection

Neuroprotection aims to safeguard neurons from damage caused by various factors like stress, potentially leading to the rescue, recovery, or regeneration of the nervous system and its functions [J Clin Neurosci. 2002;9(1):4-8]. Conversely, neuroplasticity refers to the brain's ability to adapt and change throughout life, involving structural and functional alterations in cells and synaptic transmission [Neural Plast. 2014;2014:541870]. Neuroprotection is a broad and multidisciplinary field encompassing various approaches and strategies aimed at preserving and promoting neuronal health. It is a critical area of research in neuroscience and neurology, with the potential to lead to new therapies for a wide range of neurological disorders and conditions. Neuroprotection can take various forms and may involve pharmacological agents, lifestyle modifications, or behavioral interventions. Accordingly, also the perspective and the meaning of neuroprotection differs due to different angles of interpretation. The primary interpretation is from the pharmacological point of view since the most consistent data come from this field. In addition, we will discuss also alternative, yet less considered, perspectives on neuroprotection, focusing on specific neuroprotective targets, interactions with surrounding microglia, different levels of neuroprotective effects, the reversive/adaptative dimension, and its use as anticipatory/prophylactic intervention.

D-Glutamate production by stressed Escherichia coli gives a clue for the hypothetical induction mechanism of the ALS disease

In the search for the origin of Amyotrophic Lateral Sclerosis disease (ALS), we hypothesized earlier (Monselise, 2019) that D-amino acids produced by stressed microbiome may serve as inducers of the disease development. Many examples of D-amino acid accumulation under various stress conditions were demonstrated in prokaryotic and eukaryotic cells. In this work, wild-type Escherichia coli, members of the digestive system, were subjected to carbon and nitrogen starvation stress. Using NMR and LC-MS techniques, we found for the first time that D-glutamate accumulated in the stressed bacteria but not in control cells. These results together with the existing knowledge, allow us to suggest a new insight into the pathway of ALS development: D-glutamate, produced by the stressed microbiome, induces neurobiochemical miscommunication setting on C1q of the complement system. Proving this insight may have great importance in preventive medicine of such MND modern-age diseases as ALS, Alzheimer, and Parkinson.

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

Introduction

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

Methods

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

Results

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

Discussion

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

Interactions Involving Glycine and Other Amino Acid Neurotransmitters: Focus on Transporter-Mediated Regulation of Release and Glycine-Glutamate Crosstalk

Glycine plays a pivotal role in the Central Nervous System (CNS), being a major inhibitory neurotransmitter as well as a co-agonist of Glutamate at excitatory NMDA receptors. Interactions involving Glycine and other neurotransmitters are the subject of different studies. Functional interactions among neurotransmitters include the modulation of release through release-regulating receptors but also through transporter-mediated mechanisms. Many transporter-mediated interactions involve the amino acid transmitters Glycine, Glutamate, and GABA. Different studies published during the last two decades investigated a number of transporter-mediated interactions in depth involving amino acid transmitters at the nerve terminal level in different CNS areas, providing details of mechanisms involved and suggesting pathophysiological significances. Here, this evidence is reviewed also considering additional recent information available in the literature, with a special (but not exclusive) focus on glycinergic neurotransmission and Glycine-Glutamate interactions. Some possible pharmacological implications, although partly speculative, are also discussed. Dysregulations in glycinergic and glutamatergic transmission are involved in relevant CNS pathologies. Pharmacological interventions on glycinergic targets (including receptors and transporters) are under study to develop novel therapies against serious CNS pathological states including pain, schizophrenia, epilepsy, and neurodegenerative diseases. Although with limitations, it is hoped to possibly contribute to a better understanding of the complex interactions between glycine-mediated neurotransmission and other major amino acid transmitters, also in view of the current interest in potential drugs acting on "glycinergic" targets.

Potential Link Between Exercise and N-Methyl-D-Aspartate Glutamate Receptors in Alcohol Use Disorder: Implications for Therapeutic Strategies

Alcohol use disorder (AUD) is a significant risk factor, accounting for approximately 13% of all deaths in the US. AUD not only destroys families but also causes economic losses due to reduced productivity, absenteeism, and healthcare expenses. Statistics revealing the sustained number of individuals affected by AUD over the years underscore the need for further understanding of the underlying pathophysiology to advance novel therapeutic strategies. Previous research has implicated the limbic brain regions N-methyl-D-aspartate glutamate receptors (NMDAR) in the emotional and behavioral effects of AUD. Given that aerobic exercise can modulate NMDAR activity and sensitivity to alcohol, this review presents a summary of clinical and basic science studies on NMDAR levels induced by alcohol consumption, as well as acute and protracted withdrawal, highlighting the potential role of aerobic exercise as an adjunctive therapy for AUD. Based on our findings, the utility of exercise in the modulation of reward-linked receptors and AUD may be mediated by its effects on NMDA signaling. These data support further consideration of the potential of aerobic exercise as a promising adjunctive therapy for AUD.

Stereoselective Amine-omics Using Heavy Atom Isotope Labeled l- and d-Marfey's Reagents

Biological amines and amino acids play essential roles in many biochemical processes. The chemical complexity of biological samples is challenging, and the selective identification and quantification of amines and amino acid stereoisomers would be very useful for amine-focused "amino-omics" studies. Many amines and amino acids are chiral, and their stereoisomers cannot be resolved on achiral media without chiral derivatization. In prior studies, we demonstrated the use of Marfey's reagent─a chiral derivatization reagent for amines and phenolic OH groups─for the LC-MS/MS resolution and quantification of amines and amino acid stereoisomers. In this study, a heavy atom isotope labeled Marfey's reagent approach for the stereoselective detection and quantification of amines and amino acids was developed. Heavy (13C2) l-Marfey's (Hl-Mar) and heavy (2H3) d-Marfey's (Hd-Mar) were synthesized from 13C2-l-Ala and 2H3-d-Ala, respectively. Both light and heavy Marfey's reagents were used to derivatize standard amine mixtures, which were analyzed by LC-QToF-HRMS. Aligned peak lists were comparatively analyzed by light vs heavy Mar mass differences to identify mono-, di-, and tri-Marfey's adducts and then by the retention time difference between l- and d-Mar derivatives to identify stereoisomers. This approach was then applied to identify achiral and chiral amine and amino acid components in a methicillin-resistant Staphylococcus aureus (MRSA) extract. This approach shows high analytical selectivity and reproducibility.

Deciphering the intricate linkage between the gut microbiota and Alzheimer's disease: Elucidating mechanistic pathways promising therapeutic strategies

BACKGROUND

The gut microbiome is composed of various microorganisms such as bacteria, fungi, and protozoa, and constitutes an important part of the human gut. Its composition is closely related to human health and disease. Alzheimer's disease (AD) is a neurodegenerative disease whose underlying mechanism has not been fully elucidated. Recent research has shown that there are significant differences in the gut microbiota between AD patients and healthy individuals. Changes in the composition of gut microbiota may lead to the development of harmful factors associated with AD. In addition, the gut microbiota may play a role in the development and progression of AD through the gut-brain axis. However, the exact nature of this relationship has not been fully understood.

AIMS

This review will elucidate the types and functions of gut microbiota and their relationship with AD and explore in depth the potential mechanisms of gut microbiota in the occurrence of AD and the prospects for treatment strategies.

METHODS

Reviewed literature from PubMed and Web of Science using key terminologies related to AD and the gut microbiome.

RESULTS

Research indicates that the gut microbiota can directly or indirectly influence the occurrence and progression of AD through metabolites, endotoxins, and the vagus nerve.

DISCUSSION

This review discusses the future challenges and research directions regarding the gut microbiota in AD.

CONCLUSION

While many unresolved issues remain regarding the gut microbiota and AD, the feasibility and immense potential of treating AD by modulating the gut microbiota are evident.

Blood D-serine levels correlate with aging and dopaminergic treatment in Parkinson's disease

We recently described increased D- and L-serine concentrations in the striatum of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated monkeys, the post-mortem caudate-putamen of human Parkinson's disease (PD) brains and the cerebrospinal fluid (CSF) of de novo living PD patients. However, data regarding blood D- and L-serine levels in PD are scarce. Here, we investigated whether the serum profile of D- and L-serine, as well as the other glutamate N-methyl-D-aspartate ionotropic receptor (NMDAR)-related amino acids, (i) differs between PD patients and healthy controls (HC) and (ii) correlates with clinical-demographic features and levodopa equivalent daily dose (LEDD) in PD. Eighty-three consecutive PD patients and forty-one HC were enrolled. PD cohort underwent an extensive clinical characterization. Serum levels of D- and L-serine, L-glutamate, L-glutamine, L-aspartate, L-asparagine and glycine were determined using High Performance Liquid Chromatography. In age- and sex-adjusted analyses, no differences emerged in the serum levels of D-serine, L-serine and other NMDAR-related amino acids between PD and HC. However, we found that D-serine and D-/Total serine ratio positively correlated with age in PD but not in HC, and also with PD age at onset. Moreover, we found that higher LEDD correlated with lower levels of D-serine and the other excitatory amino acids. Following these results, the addition of LEDD as covariate in the analyses disclosed a selective significant increase of D-serine in PD compared to HC (Δ ≈ 38%). Overall, these findings suggest that serum D-serine and D-/Total serine may represent a valuable biochemical signature of PD.

Age-Dependent Activation of Pannexin1 Function Contributes to the Development of Epileptogenesis in Autosomal Dominant Sleep-related Hypermotor Epilepsy Model Rats

To explore the processes of epileptogenesis/ictogenesis, this study determined the age-dependent development of the functional abnormalities in astroglial transmission associated with pannexin1-hemichannel using a genetic rat model of autosomal dominant sleep-related hypermotor epilepsy (ADSHE) named 'S286L-TG'. Pannexin1 expression in the plasma membrane of primary cultured cortical astrocytes and the orbitofrontal cortex (OFC), which is an ADSHE focus region, were determined using capillary immunoblotting. Astroglial D-serine releases induced by artificial high-frequency oscillation (HFO)-evoked stimulation, the removal of extracellular Ca2+, and the P2X7 receptor agonist (BzATP) were determined using ultra-high performance liquid chromatography (UHPLC). The expressions of pannexin1 in the plasma membrane fraction of the OFC in S286L-TG at four weeks old were almost equivalent when compared to the wild type. The pannexin1 expression in the OFC of the wild type non-statistically decreased age-dependently, whereas that in S286L-TG significantly increased age-dependently, resulting in relatively increasing pannexin1 expression from the 7- (at the onset of interictal discharge) and 10-week-old (after the ADSHE seizure onset) S286L-TG compared to the wild type. However, no functional abnormalities of astroglial pannexin1 expression or D-serine release through the pannexin1-hemichannels from the cultured astrocytes of S286L-TG could be detected. Acutely HFO-evoked stimulation, such as physiological ripple burst (200 Hz) and epileptogenic fast ripple burst (500 Hz), frequency-dependently increased both pannexin1 expression in the astroglial plasma membrane and astroglial D-serine release. Neither the selective inhibitors of pannexin1-hemichannel (10PANX) nor connexin43-hemichannel (Gap19) affected astroglial D-serine release during the resting stage, whereas HFO-evoked D-serine release was suppressed by both inhibitors. The inhibitory effect of 10PANX on the ripple burst-evoked D-serine release was more predominant than that of Gap19, whereas fast ripple burst-evoked D-serine release was predominantly suppressed by Gap19 rather than 10PANX. Astroglial D-serine release induced by acute exposure to BzATP was suppressed by 10PANX but not by Gap19. These results suggest that physiological ripple burst during the sleep spindle plays important roles in the organization of some components of cognition in healthy individuals, but conversely, it contributes to the initial development of epileptogenesis/ictogenesis in individuals who have ADSHE vulnerability via activation of the astroglial excitatory transmission associated with pannexin1-hemichannels.