d-serine levels in Alzheimer's disease: implications for novel biomarker development

Neurobiology of L-proline: From molecules to behavior

L-proline is an amino acid with a unique cyclic structure, involvement in various physiological processes, such as protein synthesis, collagen production, and neurotransmission. This review explores the complex roles of proline in the central nervous system (CNS), where it contributes to both excitatory and inhibitory neurotransmission. Additionally, L-proline has distinct metabolic functions attributed to its structural properties. The concentration-dependent effects of L-proline indicate its importance in CNS function, with potential implications for health and disease. Studies in animal models suggest that L-proline influences cognitive function and behavior, with dysregulated levels linked to learning and memory deficits. Furthermore, this review addresses the neuropathological consequences of hyperprolinemia, a metabolic disorder marked by elevated L-proline levels in the CNS and examines the potential role of L-proline in neurological and psychiatric disorders. In sum, this work provides a comprehensive perspective on the neurobiological importance of L-proline, underscoring its involvement in neurotransmission, behavioral modulation, and disease pathology.

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 and challenges in serine in the central nervous system: physicochemistry, physiology, and pharmacology

Neurological disorders are the primary cause of human disability and mortality globally, however, current medications slightly alleviate some symptoms of degenerative diseases. Serine is an important amino acid for the brain function and involved in a variety of biosynthetic pathways and signal transduction processes. The imbalance of serine metabolism is associated with neurodegeneration, including neuroinflammation, oxidative stress and apoptosis. Altered activities of serine metabolizing enzymes and accumulation of serine metabolites affect the survival and function of nerve cells. Abnormal serine levels are observed in animal models with neurological diseases, but not all human studies, therefore, the maintenance of serine homeostasis is a potentially therapeutic strategy for neurological disorders. To date, physiological and pharmacological roles of serine in neurological diseases have not been systemically recapitulated, and the association between serine and neurological diseases is controversial. In this review, we summarize physicochemical properties of serine, biological processes of serine in the brain (source, biotransformation, and transport), and the application of serine in neurological diseases including Alzheimer's disease, schizophrenia, and depression. Here, we highlight physicochemistry, physiology, pharmacology, and therapeutic potentials of serine in the prevention and treatment of neurological dysfunction. Our work provides valuable hints for future investigation that will lead to a comprehensive understanding of serine and its metabolism in cellular physiology and pharmacology. Although broad by necessity, the review helps researchers to understand great potentials of serine in the prevention and treatment of neurological dysfunction.

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.

Characterisation of a microelectrochemical biosensor for real-time detection of brain extracellular d-serine

A modified development protocol and concomitant characterisation of a first generation biosensor for the detection of brain extracellular d-serine is reported. Functional parameters important for neurochemical monitoring, including sensor sensitivity, O2 interference, selectivity, shelf-life and biocompatibility were examined. Construction and development involved the enzyme d-amino acid oxidase (DAAO), utilising a dip-coating immobilisation method employing a new extended drying approach. The resultant Pt-based polymer enzyme composite sensor achieved high sensitivity to d-serine (0.76 ± 0.04 nA mm-2. μM-1) and a low μM limit of detection (0.33 ± 0.02 μM). The in-vitro response time was within the solution stirring time, suggesting potential sub-second in-vivo response characteristics. Oxygen interference studies demonstrated a 1 % reduction in current at 50 μM O2 when compared to atmospheric O2 levels (200 μM), indicating that the sensor can be used for reliable neurochemical monitoring of d-serine, free from changes in current associated with physiological O2 fluctuations. Potential interference signals generated by the principal electroactive analytes present in the brain were minimised by using a permselective layer of poly(o-phenylenediamine), and although several d-amino acids are possible substrates for DAAO, their physiologically relevant signals were small relative to that for d-serine. Additionally, changing both temperature and pH over possible in vivo ranges (34-40 °C and 7.2-7.6 respectively) resulted in no significant effect on performance. Finally, the biosensor was implanted in the striatum of freely moving rats and used to monitor physiological changes in d-serine over a two-week period.

Serum dysregulation of serine and glycine metabolism as predictive biomarker for cognitive decline in frail elderly subjects

Frailty is a common age-related clinical syndrome characterized by a decline in the function of multiple organ systems, increased vulnerability to stressors, and a huge socio-economic burden. Despite recent research efforts, the physiopathological mechanisms underlying frailty remain elusive and biomarkers able to predate its occurrence in the early stages are still lacking. Beyond its physical component, cognitive decline represents a critical domain of frailty associated with higher risk of adverse health outcomes. We measured by High-Performance Liquid Chromatography (HPLC) a pool of serum amino acids including L-glutamate, L-aspartate, glycine, and D-serine, as well as their precursors L-glutamine, L-asparagine, and L-serine in a cohort of elderly subjects encompassing the entire continuum from fitness to frailty. These amino acids are known to orchestrate excitatory and inhibitory neurotransmission, and in turn, to play a key role as intermediates of energy homeostasis and in liver, kidney, muscle, and immune system metabolism. To comprehensively assess frailty, we employed both the Edmonton Frail Scale (EFS), as a practical tool to capture the multidimensionality of frailty, and the frailty phenotype, as a measure of physical function. We found that D-serine and D-/Total serine ratio were independent predictors of EFS but not of physical frailty. Furthermore, higher levels of glycine, glycine/L-serine and D-/Total serine were associated with worse cognition and depressive symptoms in the frail group. These findings suggest that changes in peripheral glycine and serine enantiomers homeostasis may represent a novel biochemical correlate of frailty.

CSF amino acid profiles in ICV-streptozotocin-induced sporadic Alzheimer's disease in male Wistar rat: a metabolomics and systems biology perspective

Alzheimer's disease (AD) is an increasingly important public health concern due to the increasing proportion of older individuals within the general population. The impairment of processes responsible for adequate brain energy supply primarily determines the early features of the aging process. Restricting brain energy supply results in brain hypometabolism prior to clinical symptoms and is anatomically and functionally associated with cognitive impairment. The present study investigated changes in metabolic profiles induced by intracerebroventricular-streptozotocin (ICV-STZ) in an AD-like animal model. To this end, male Wistar rats received a single injection of STZ (3 mg·kg-1) by ICV (2.5 μL into each ventricle for 5 min on each side). In the second week after receiving ICV-STZ, rats were tested for cognitive performance using the Morris Water Maze test and subsequently prepared for positron emission tomography (PET) to confirm AD-like symptoms. Tandem Mass Spectrometry (MS/MS) analysis was used to detect amino acid changes in cerebrospinal fluid (CFS) samples. Our metabolomics study revealed a reduction in the concentrations of various amino acids (alanine, arginine, aspartic acid, glutamic acid, glycine, isoleucine, methionine, phenylalanine, proline, serine, threonine, tryptophane, tyrosine, and valine) in CSF of ICV-STZ-treated animals as compared to controls rats. The results of the current study indicate amino acid levels could potentially be considered targets of nutritional and/or pharmacological interventions to interfere with AD progression.

The Development of a Regulator of Human Serine Racemase for N-Methyl-D-aspartate Function

It is crucial to regulate N-methyl-D-aspartate (NMDA) function bivalently depending on the central nervous system (CNS) conditions. CNS disorders with NMDA hyperfunction are involved in the pathogenesis of neurotoxic and/or neurodegenerative disorders with elevated D-serine, one of the NMDA receptor co-agonists. On the contrary, NMDA-enhancing agents have been demonstrated to improve psychotic symptoms and cognition in CNS disorders with NMDA hypofunction. Serine racemase (SR), the enzyme regulating both D- and L-serine levels through both racemization (catalysis from L-serine to D-serine) and β-elimination (degradation of both D- and L-serine), emerges as a promising target for bidirectional regulation of NMDA function. In this study, we explored using dimethyl malonate (DMM), a pro-drug of the SR inhibitor malonate, to modulate NMDA activity in C57BL/6J male mice via intravenous administration. Unexpectedly, 400 mg/kg DMM significantly elevated, rather than decreased (as a racemization inhibitor), D-serine levels in the cerebral cortex and plasma. This outcome prompted us to investigate the regulatory effects of dodecagalloyl-α-D-xylose (α12G), a synthesized tannic acid analog, on SR activity. Our findings showed that α12G enhanced the racemization activity of human SR by about 8-fold. The simulated and fluorescent assay of binding affinity suggested a noncooperative binding close to the catalytic residues, Lys56 and Ser84. Moreover, α12G treatment can improve behaviors associated with major CNS disorders with NMDA hypofunction including hyperactivity, prepulse inhibition deficit, and memory impairment in animal models of positive symptoms and cognitive impairment of psychosis. In sum, our findings suggested α12G is a potential therapeutic for treating CNS disorders with NMDA hypofunction.

A comprehensive review on signaling attributes of serine and serine metabolism in health and disease

Serine is a metabolite with ever-expanding metabolic and non-metabolic signaling attributes. By providing one‑carbon units for macromolecule biosynthesis and functional modifications, serine and serine metabolism largely impinge on cellular survival and function. Cancer cells frequently have a preference for serine metabolic reprogramming to create a conducive metabolic state for survival and aggressiveness, making intervention of cancer-associated rewiring of serine metabolism a promising therapeutic strategy for cancer treatment. Beyond providing methyl donors for methylation in modulation of innate immunity, serine metabolism generates formyl donors for mitochondrial tRNA formylation which is required for mitochondrial function. Interestingly, fully developed neurons lack the machinery for serine biosynthesis and rely heavily on astrocytic l-serine for production of d-serine to shape synaptic plasticity. Here, we recapitulate recent discoveries that address the medical significance of serine and serine metabolism in malignancies, mitochondrial-associated disorders, and neurodegenerative pathologies. Metabolic control and epigenetic- and posttranslational regulation of serine metabolism are also discussed. Given the metabolic similarities between cancer cells, neurons and germ cells, we further propose the relevance of serine metabolism in testicular homeostasis. Our work provides valuable hints for future investigations that will lead to a deeper understanding of serine and serine metabolism in cellular physiology and pathology.

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.

Lysophosphatidylcholines are associated with P-tau181 levels in early stages of Alzheimer's Disease

Background

We profiled circulating plasma metabolites to identify systemic biochemical changes in clinical and biomarker-assisted diagnosis of Alzheimer's disease (AD).

Methods

We used an untargeted approach with liquid chromatography coupled to high-resolution mass spectrometry to measure small molecule plasma metabolites from 150 clinically diagnosed AD patients and 567 age-matched healthy elderly of Caribbean Hispanic ancestry. Plasma biomarkers of AD were measured including P-tau181, Aβ40, Aβ42, total-tau, neurofilament light chain (NfL) and glial fibrillary acidic protein (GFAP). Association of individual and co-abundant modules of metabolites were tested with clinical diagnosis of AD, as well as biologically-defined AD pathological process based on P-tau181 and other biomarker levels.

Results

Over 6000 metabolomic features were measured with high accuracy. First principal component (PC) of lysophosphatidylcholines (lysoPC) that bind to or interact with docosahexaenoic acid (DHA), eicosapentaenoic acid (EPA) and arachidonic acid (AHA) was associated with decreased risk of AD (OR = 0.91 [0.89-0.96], p = 2e-04). Association was restricted to individuals without an APOE ε4 allele (OR = 0.89 [0.84-0.94], p = 8.7e-05). Among individuals carrying at least one APOE ε4 allele, PC4 of lysoPCs moderately increased risk of AD (OR = 1.37 [1.16-1.6], p = 1e-04). Essential amino acids including tyrosine metabolism pathways were enriched among metabolites associated with P-tau181 levels and heparan and keratan sulfate degradation pathways were associated with Aβ42/Aβ40 ratio.

Conclusions

Unbiased metabolic profiling can identify critical metabolites and pathways associated with β-amyloid and phosphotau pathology. We also observed an APOE-ε4 dependent association of lysoPCs with AD and biologically based diagnostic criteria may aid in the identification of unique pathogenic mechanisms.

Comprehensive Analysis of Metabolites in Postmortem Brains of Patients with Alzheimer's Disease

BACKGROUND

Disturbed metabolism has been proposed as being involved in the pathogenesis of Alzheimer's disease (AD), and more evidence from human AD brains is required.

OBJECTIVE

In this study, we attempted to identify or confirm modulations in the levels of metabolites associated with AD in postmortem AD brains.

METHODS

We performed metabolomics analyses using a gas chromatography mass spectrometry system in postmortem brains of patients with confirmed AD, patients with CERAD score B, and control subjects.

RESULTS

Impaired phosphorylation of glucose and elevation of several tricarboxylic acid (TCA) metabolites, except citrate, were observed and the degree of impaired phosphorylation and elevation in the levels of the TCA cycle metabolites were negatively and positively correlated, respectively, with the clinical phenotypes of AD. The levels of uronic acid pathway metabolites were modulated in AD and correlated positively with the amyloid-β content. The associations of nucleic acid synthesis and amino acid metabolites with AD depended on the kinds of metabolites; in particular, the contents of ribose 5-phosphate, serine and glycine were negatively correlated, while those of ureidosuccinic acid and indole-3-acetic acid were positively modulated in AD. Comprehensive statistical analyses suggested that alterations in the inositol pathway were most closely associated with AD.

CONCLUSIONS

The present study revealed many novel associations between metabolites and AD, suggesting that some of these might serve as novel potential therapeutic targets for AD.

Fabrication of a polymer brush-functionalized porphyrin-based covalent organic framework for enrichment of N-glycopeptides

A surface-initiated atom transfer radical polymerization method combining click chemistry was employed to prepare a novel porphyrin-based covalent organic framework composite grafted with polymer brushes (TAPBB@GMA@AMA@Cys) for the specific enrichment of N-glycopeptides. The material successfully realized the high efficiency enrichment of N-glycopeptides with good selectivity (1:1000), low detection limit (0.2 fmol/μL), and high loading capacity (133.3 mg·g-1). The TAPBB@GMA@AMA@Cys was successfully applied to actual sample analysis; 235 N-glycopeptides related to 125 glycoproteins and 210 N-glycopeptides related to 121 glycoproteins were recognized from the serum of normal individuals and Alzheimer's disease patients, respectively. Gene ontology studies of molecular functions, cellular components, and biological processes have revealed that identified glycoproteins are strongly associated with neurodegenerative diseases involving innate immune responses, basement membranes, calcium binding, and receptor binding. The above results confirm the surprising potential of materials in glycoproteomics research and practical sample applications.

Mechanisms of NMDA receptor regulation

N-methyl-D-aspartate receptors (NMDARs) are glutamate-gated ion channels widely expressed in the central nervous system that play key role in brain development and plasticity. On the downside, NMDAR dysfunction, be it hyperactivity or hypofunction, is harmful to neuronal function and has emerged as a common theme in various neuropsychiatric disorders including autism spectrum disorders, epilepsy, intellectual disability, and schizophrenia. Not surprisingly, NMDAR signaling is under a complex set of regulatory mechanisms that maintain NMDAR-mediated transmission in check. These include an unusual large number of endogenous agents that directly bind NMDARs and tune their activity in a subunit-dependent manner. Here, we review current knowledge on the regulation of NMDAR signaling. We focus on the regulation of the receptor by its microenvironment as well as by external (i.e. pharmacological) factors and their underlying molecular and cellular mechanisms. Recent developments showing how NMDAR dysregulation participate to disease mechanisms are also highlighted.

Synaptic proteasome is inhibited in Alzheimer's disease models and associates with memory impairment in mice

The proteasome plays key roles in synaptic plasticity and memory by regulating protein turnover, quality control, and elimination of oxidized/misfolded proteins. Here, we investigate proteasome function and localization at synapses in Alzheimer's disease (AD) post-mortem brain tissue and in experimental models. We found a marked increase in ubiquitinylated proteins in post-mortem AD hippocampi compared to controls. Using several experimental models, we show that amyloid-β oligomers (AβOs) inhibit synaptic proteasome activity and trigger a reduction in synaptic proteasome content. We further show proteasome inhibition specifically in hippocampal synaptic fractions derived from APPswePS1ΔE9 mice. Reduced synaptic proteasome activity instigated by AβOs is corrected by treatment with rolipram, a phosphodiesterase-4 inhibitor, in mice. Results further show that dynein inhibition blocks AβO-induced reduction in dendritic proteasome content in hippocampal neurons. Finally, proteasome inhibition induces AD-like pathological features, including reactive oxygen species and dendritic spine loss in hippocampal neurons, inhibition of hippocampal mRNA translation, and memory impairment in mice. Results suggest that proteasome inhibition may contribute to synaptic and memory deficits in AD.

Proteomic alterations in the brain and blood-brain barrier during brain Aβ accumulation in an APP knock-in mouse model of Alzheimer's disease

BACKGROUND

Blood-brain barrier (BBB) dysfunction is supposed to be an early event in the development of Alzheimer's disease (AD). This study aimed to investigate the relationship between BBB alterations and AD progression in terms of amyloid-β peptide (Aβ) accumulation in the brains of humanized amyloid precursor protein knock-in (APP-KI) mice.

METHODS

Brain Aβ accumulation was examined using immunohistochemical analysis. Alterations in differentially expressed proteins were determined using sequential window acquisition of all theoretical fragment ion mass spectroscopy (SWATH-MS)-based quantitative proteomics, and Metascape, STRING, Gene Ontology, and KEGG were used for network analyses of altered biological pathways and processes. Statistical significance was determined using the unpaired two-tailed Student's t-test and Welch's t-test for two groups and one-way analysis of variance followed by Tukey's test for more than two groups. Correlations between two groups were determined using Pearson's correlation analysis.

RESULTS

Brain Aβ accumulation in APP-KI mice was detectable at 2 months, increased significantly at 5 months, and remained elevated at 12 months of age. The levels of differentially expressed proteins in isolated brain capillaries were higher in younger mice, whereas those in the brain were higher in older mice. Network analyses indicated changes in basement membrane-associated and ribosomal proteins in the brain capillaries. There were no significant changes in key proteins involved in drug or Aβ transport at the BBB. In contrast, solute carrier transporter levels in astrocytes, microglia, and neurons were altered in the brain of older mice. Moreover, the levels of the lipid transporters Apoe and Apoj were upregulated in both the brain and isolated brain capillaries after Aβ accumulation.

CONCLUSIONS

Our results suggest that changes in the brain occurred after advanced Aβ accumulation, whereas initial Aβ accumulation was sufficient to cause alterations in the BBB. These findings may help elucidate the role of BBB alterations in AD progression and predict the distribution of drugs across the BBB in the brain of patients with AD.

19F NMR enantiodiscrimination and diastereomeric purity determination of amino acids, dipeptides, and amines

Chiral amino-group compounds are of significance for human health, such as biogenic amino acids (AAs), dipeptides, and even various drugs. Enantiospecific discrimination of these chiral compounds is vital in diagnosing diseases, identifying pathological biomarkers and enhancing pharmaceutical chemistry research. Here, we report a simple and rapid 19F NMR-based strategy to differentiate chiral AAs, dipeptides, and amines, that were derivatized with (R)-2-(2-fluorophenyl)-2-hydroxyacetic acid ((R)-2FHA). As a result, 19 proteinogenic AAs (37 isomers) as well as Gly could be concurrently resolved. Moreover, various mirror-image dipeptides, such as Ser-His, Leu-Leu, and Ala-Ala, were commendably recognized. Intriguingly, we found that the absolute configuration of AAs in the N-terminus of dipeptides decided the relative 19F chemical shifts between two enantiomers. Besides, the ability of this method for enantiodiscrimination was further demonstrated by non-AA amines, including aromatic and aliphatic amines, and even amines having chiral centers several carbons away from the amino-group. The structurally similar antibiotics, amoxicillin and ampicillin, were well discriminated. Furthermore, this method accurately determines the de or dr values of non-racemic mixtures. Therefore, our strategy provides an effective approach for 19F NMR-based enantiodiscrimination and diastereomeric purity determination of amino-group compounds.

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.

Emerging concepts towards a translational framework in Alzheimer's disease

Over the past decades, significant efforts have been made to understand the precise mechanisms underlying the pathogenesis of Alzheimer's disease (AD), the most common cause of dementia. However, clinical trials targeting AD pathological hallmarks have consistently failed. Refinement of AD conceptualization, modeling, and assessment is key to developing successful therapies. Here, we review critical findings and discuss emerging ideas to integrate molecular mechanisms and clinical approaches in AD. We further propose a refined workflow for animal studies incorporating multimodal biomarkers used in clinical studies - delineating critical paths for drug discovery and translation. Addressing unresolved questions with the proposed conceptual and experimental framework may accelerate the development of effective disease-modifying strategies for AD.

Lysophosphatidylcholines are associated with P-tau181 levels in early stages of Alzheimer's Disease

Background

We investigated systemic biochemical changes in Alzheimer's disease (AD) by investigating the relationship between circulating plasma metabolites and both clinical and biomarker-assisted diagnosis of AD.

Methods

We used an untargeted approach with liquid chromatography coupled to high-resolution mass spectrometry to measure exogenous and endogenous small molecule metabolites in plasma from 150 individuals clinically diagnosed with AD and 567 age-matched elderly without dementia of Caribbean Hispanic ancestry. Plasma biomarkers of AD were also measured including P-tau181, Aβ40, Aβ42, total tau, neurofilament light chain (NfL) and glial fibrillary acidic protein (GFAP). Association of individual and co-expressed modules of metabolites were tested with the clinical diagnosis of AD, as well as biologically-defined AD pathological process based on P-tau181 and other biomarker levels.

Results

Over 4000 metabolomic features were measured with high accuracy. First principal component (PC) of lysophosphatidylcholines (lysoPC) that bind to or interact with docosahexaenoic acid (DHA), eicosapentaenoic acid (EPA) and arachidonic acid (AHA) was associated with decreased risk of AD (OR=0.91 [0.89-0.96], p=2e-04). Restricted to individuals without an APOE ε4 allele (OR=0.89 [0.84-0.94], p= 8.7e-05), the association remained. Among individuals carrying at least one APOE ε4 allele, PC4 of lysoPCs moderately increased risk of AD (OR=1.37 [1.16-1.6], p=1e-04). Essential amino acids including tyrosine metabolism pathways were enriched among metabolites associated with P-tau181 levels and heparan and keratan sulfate degradation pathways were associated with Aβ42/Aβ40 ratio reflecting different pathways enriched in early and middle stages of disease.

Conclusions

Our findings indicate that unbiased metabolic profiling can identify critical metabolites and pathways associated with β-amyloid and phosphotau pathology. We also observed an APOE ε4 dependent association of lysoPCs with AD and that biologically-based diagnostic criteria may aid in the identification of unique pathogenic mechanisms.

Chirality in Light-Matter Interaction

The scientific effort to control the interaction between light and matter has grown exponentially in the last 2 decades. This growth has been aided by the development of scientific and technological tools enabling the manipulation of light at deeply sub-wavelength scales, unlocking a large variety of novel phenomena spanning traditionally distant research areas. Here, the role of chirality in light-matter interactions is reviewed by providing a broad overview of its properties, materials, and applications. A perspective on future developments is highlighted, including the growing role of machine learning in designing advanced chiroptical materials to enhance and control light-matter interactions across several scales.

The unfolded protein response transcription factor XBP1s ameliorates Alzheimer's disease by improving synaptic function and proteostasis

Alteration in the buffering capacity of the proteostasis network is an emerging feature of Alzheimer's disease (AD), highlighting the occurrence of endoplasmic reticulum (ER) stress. The unfolded protein response (UPR) is the main adaptive pathway to cope with protein folding stress at the ER. Inositol-requiring enzyme-1 (IRE1) operates as a central ER stress sensor, enabling the establishment of adaptive and repair programs through the control of the expression of the transcription factor X-box binding protein 1 (XBP1). To artificially enforce the adaptive capacity of the UPR in the AD brain, we developed strategies to express the active form of XBP1 in the brain. Overexpression of XBP1 in the nervous system using transgenic mice reduced the load of amyloid deposits and preserved synaptic and cognitive function. Moreover, local delivery of XBP1 into the hippocampus of an 5xFAD mice using adeno-associated vectors improved different AD features. XBP1 expression corrected a large proportion of the proteomic alterations observed in the AD model, restoring the levels of several synaptic proteins and factors involved in actin cytoskeleton regulation and axonal growth. Our results illustrate the therapeutic potential of targeting UPR-dependent gene expression programs as a strategy to ameliorate AD features and sustain synaptic function.

Regional contributions of D-serine to Alzheimer's disease pathology in male AppNL-G-F/NL-G-F mice

Background

Neurodegenerative processes in Alzheimer's disease (AD) are associated with excitotoxicity mediated by the N-methyl-D-aspartate receptor (NMDAR). D-Serine is an endogenous co-agonist necessary for NMDAR-mediated excitotoxicity. In the mammalian brain, it is produced by serine racemase (SRR) from L-serine, suggesting that dysregulation of L-serine, D-serine, or SRR may contribute to AD pathogenesis.

Objective and methods

We examined the contributions of D-serine to AD pathology in the App^{NL-G-F/NL-G-F} gene knock-in (APPKI) mouse model of AD. We first examined brain SRR expression levels and neuropathology in APPKI mice and then assessed the effects of long-term D-serine supplementation in drinking water on neurodegeneration. To further confirm the involvement of endogenous D-serine in AD progression, we generated Srr gene-deleted APPKI (APPKI-SRRKO) mice. Finally, to examine the levels of brain amino acids, we conducted liquid chromatography-tandem mass spectrometry.

Results

Expression of SRR was markedly reduced in the retrosplenial cortex (RSC) of APPKI mice at 12 months of age compared with age-matched wild-type mice. Neuronal density was decreased in the hippocampal CA1 region but not altered significantly in the RSC. D-Serine supplementation exacerbated neuronal loss in the hippocampal CA1 of APPKI mice, while APPKI-SRRKO mice exhibited attenuated astrogliosis and reduced neuronal death in the hippocampal CA1 compared with APPKI mice. Furthermore, APPKI mice demonstrated marked abnormalities in the cortical amino acid levels that were partially reversed in APPKI-SRRKO mice.

Conclusion

These findings suggest that D-serine participates in the regional neurodegenerative process in the hippocampal CA1 during the amyloid pathology of AD and that reducing brain D-serine can partially attenuate neuronal loss and reactive astrogliosis. Therefore, regulating SRR could be an effective strategy to mitigate NMDAR-dependent neurodegeneration during AD progression.

Homeostasis of serine enantiomers is disrupted in the post-mortem caudate putamen and cerebrospinal fluid of living Parkinson's disease patients

L-serine generated in astrocytes plays a pivotal role in modulating essential neurometabolic processes, while its enantiomer, D-serine, specifically regulates NMDA receptor (NMDAR) signalling. Despite their physiological relevance in modulating cerebral activity, serine enantiomers metabolism in Parkinson's disease (PD) remains elusive. Using High-Performance Liquid Chromatography (HPLC), we measured D- and L-serine levels along with other amino acids known to modulate NMDAR function, such as L-glutamate, L-aspartate, D-aspartate, and glycine, in the post-mortem caudate putamen (CPu) and superior frontal gyrus (SFG) of PD patients. Moreover, we examined these amino acids in the cerebrospinal fluid (CSF) of de novo living PD, Alzheimer's disease (AD), and amyotrophic lateral sclerosis (ALS) patients versus subjects with other neurological disorders (OND), used as control. We found higher D-serine and L-serine levels in the CPu of PD patients but not in the SFG, a cerebral region that, in contrast to the CPu, is not innervated by nigral dopaminergic terminals. We also highlighted a significant elevation of both serine enantiomers in the CSF samples from PD but not in those of AD and ALS patients, compared with control subjects. By contrast, none or only minor changes were found in the amount of other neuroactive amino acids mentioned above. Our findings identify D-serine and L-serine level upregulation as a biochemical signature associated with nigrostriatal dopaminergic degeneration in PD.

Differential relationships of NMDAR hypofunction and oxidative stress with cognitive decline

NMDAR hypofunction and oxidative stress are implicated in the pathogenesis of Alzheimer's disease. D-amino acid oxidase (DAO) regulates NMDAR function. Glutathione, superoxide dismutase, and catalase are three first-line endogenous antioxidants. This study explored the associations of these potential biomarkers with mild cognitive impairment. Cognitive function and blood levels of DAO, glutathione, superoxide dismutase, and catalase were measured in 63 mild cognitive impairment patients and 24 healthy individuals every 6 months for 2 years. Among the patients, DAO and glutathione levels at baseline contributed to the cognitive decline 2 years later. Among the healthy individuals, only glutathione levels were associated with cognitive change. The four biomarkers differed in change directions (upward vs. downward) in the patients and in the healthy individuals. Among patients, glutathione levels were negatively correlated with superoxide dismutase and positively correlated with catalase, and DAO levels were negatively correlated with superoxide dismutase. To our knowledge, this is the first study to demonstrate the differential associations of NMDAR hypofunction and oxidative stress with cognitive change between the mild cognitive impairment patients and healthy people. Glutathione may be regarded as an aging marker for both mild cognitive impairment and normal aging; and DAO, a biomarker exclusively for mild cognitive impairment.

Detection and analysis of chiral molecules as disease biomarkers

The chirality of small metabolic molecules is important in controlling physiological processes and indicating the health status of humans. Abnormal enantiomeric ratios of chiral molecules in biofluids and tissues occur in many diseases, including cancers and kidney and brain diseases. Thus, chiral small molecules are promising biomarkers for disease diagnosis, prognosis, adverse drug-effect monitoring, pharmacodynamic studies and personalized medicine. However, it remains difficult to achieve cost-effective and reliable analysis of small chiral molecules in clinical procedures, in part owing to their large variety and low concentration. In this Review, we describe current and emerging techniques that detect and quantify small-molecule enantiomers and their biological importance.

Promising applications of D-amino acids in periprosthetic joint infection

Due to the rise in our aging population, a disproportionate demand for total joint arthroplasty (TJA) in the elderly is forecast. Periprosthetic joint infection (PJI) represents one of the most challenging complications that can occur following TJA, and as the number of primary and revision TJAs continues to rise, an increasing PJI burden is projected. Despite advances in operating room sterility, antiseptic protocols, and surgical techniques, approaches to prevent and treat PJI remain difficult, primarily due to the formation of microbial biofilms. This difficulty motivates researchers to continue searching for an effective antimicrobial strategy. The dextrorotatory-isoforms of amino acids (D-AAs) are essential components of peptidoglycan within the bacterial cell wall, providing strength and structural integrity in a diverse range of species. Among many tasks, D-AAs regulate cell morphology, spore germination, and bacterial survival, evasion, subversion, and adhesion in the host immune system. When administered exogenously, accumulating data have demonstrated that D-AAs play a pivotal role against bacterial adhesion to abiotic surfaces and subsequent biofilm formation; furthermore, D-AAs have substantial efficacy in promoting biofilm disassembly. This presents D-AAs as promising and novel targets for future therapeutic approaches. Despite their emerging antibacterial efficacy, their role in disrupting PJI biofilm formation, the disassembly of established TJA biofilm, and the host bone tissue response remains largely unexplored. This review aims to examine the role of D-AAs in the context of TJAs. Data to date suggest that D-AA bioengineering may serve as a promising future strategy in the prevention and treatment of PJI.

The identification of a potential plasma metabolite marker for Alzheimer’s disease by LC-MS untargeted metabolomics

BACKGROUND AND AIMS

Alzheimer's disease (AD), the most common type of dementia, is hard to recognize early, resulting in delayed treatment and poor outcome. At present, there is neither reliable, non-invasive methods to diagnose it accurately and nor effective drugs to recover it. Discovery and quantification of novel metabolite markers in plasma of AD patients and investigation of the correlation between the markers and AD assessment scores.

MATERIALS AND METHODS

Untargeted liquid chromatography-mass spectrometry (LC-MS)-based metabolomics with LC-quadrupole- time-of-flight (Q-TOF) was performed in plasma samples of age-matched AD patients and healthy controls. The potential markers were further quantified with targeted multiple reaction monitoring (MRM) approach.

RESULTS

Among the candidates, progesterone, and 3-indoleacetic acid (3-IAA) were successfully identified and then validated in 50 plasma samples from 25 AD patients and 25 matched normal controls with MRM approach. As a result, 3-IAA was significantly altered in AD patients and correlated with some AD assessment scores.

CONCLUSION

By using untargeted LC-MS metabolomic and LC-MRM approaches to analyze plasma metabolites of AD patients and normal subjects, 3-IAA was discovered and quantified to be significantly altered in AD patients and correlated with several AD assessment scores.

Promoting Endogenous Neurogenesis as a Treatment for Alzheimer's Disease

Alzheimer's disease (AD) is the most universal neurodegenerative disorder characterized by memory loss and cognitive impairment. AD is biologically defined by production and aggregation of misfolded protein including extracellular amyloid β (Aβ) peptide and intracellular microtubule-associated protein tau tangles in neurons, leading to irreversible neuronal loss. At present, regulation of endogenous neurogenesis to supplement lost neurons has been proposed as a promising strategy for treatment of AD. However, the exact underlying mechanisms of impaired neurogenesis in AD have not been fully explained and effective treatments targeting neurogenesis for AD are limited. In this review, we mainly focus on the latest research of impaired neurogenesis in AD. Then we discuss the factors affecting stages of neurogenesis and the interplay between neural stem cells (NSCs) and neurogenic niche under AD pathological conditions. This review aims to explore potential therapeutic strategies that promote endogenous neurogenesis for AD treatments.

Information-Rich, Dual-Function 13C/2H-Isotopic Crosstalk NMR Assay for Human Serine Racemase (hSR) Provides a PLP-Enzyme "Partitioning Fingerprint" and Reveals Disparate Chemotypes for hSR Inhibition

The first dual-function assay for human serine racemase (hSR), the only bona fide racemase in human biology, is reported. The hSR racemization function is essential for neuronal signaling, as the product, d-serine (d-Ser), is a potent N-methyl d-aspartate (NMDA) coagonist, important for learning and memory, with dysfunctional d-Ser-signaling being observed in some neuronal disorders. The second hSR function is β-elimination and gives pyruvate; this activity is elevated in colorectal cancer. This new NMR-based assay allows one to monitor both α-proton-exchange chemistry and β-elimination using only the native l-Ser substrate and hSR and is the most sensitive such assay. The assay judiciously employs segregated dual 13C-labeling and 13C/2H crosstalk, exploiting both the splitting and shielding effects of deuterium. The assay is deployed to screen a 1020-compound library and identifies an indolo-chroman-2,4-dione inhibitor family that displays allosteric site binding behavior (noncompetitive inhibition vs l-Ser substrate; competitive inhibition vs adenosine 5'-triphosphate (ATP)). This assay also reveals important mechanistic information for hSR; namely, that H/D exchange is ∼13-fold faster than racemization, implying that K56 protonates the carbanionic intermediate on the si-face much faster than does S84 on the re-face. Moreover, the 13C NMR peak pattern seen is suggestive of internal return, pointing to K56 as the likely enamine-protonating residue for β-elimination. The 13C/2H-isotopic crosstalk assay has also been applied to the enzyme tryptophan synthase and reveals a dramatically different partition ratio in this active site (β-replacement: si-face protonation ∼6:1 vs β-elimination: si-face protonation ∼1:3.6 for hSR), highlighting the value of this approach for fingerprinting the pyridoxal phosphate (PLP) enzyme mechanism.

Biomarkers for dementia in Latin American countries: Gaps and opportunities

Limited knowledge on dementia biomarkers in Latin American and Caribbean (LAC) countries remains a serious barrier. Here, we reported a survey to explore the ongoing work, needs, interests, potential barriers, and opportunities for future studies related to biomarkers. The results show that neuroimaging is the most used biomarker (73%), followed by genetic studies (40%), peripheral fluids biomarkers (31%), and cerebrospinal fluid biomarkers (29%). Regarding barriers in LAC, lack of funding appears to undermine the implementation of biomarkers in clinical or research settings, followed by insufficient infrastructure and training. The survey revealed that despite the above barriers, the region holds a great potential to advance dementia biomarkers research. Considering the unique contributions that LAC could make to this growing field, we highlight the urgent need to expand biomarker research. These insights allowed us to propose an action plan that addresses the recommendations for a biomarker framework recently proposed by regional experts.

Biomarkers of Activity-Dependent Plasticity and Persistent Enhancement of Synaptic Transmission in Alzheimer Disease: A Review of the Current Status

Alzheimer disease (AD) is a chronic and heterogeneous neurodegenerative disorder characterized by complex pathological processes involving neuroinflammation, neurodegeneration, and synaptic dysfunction. Understanding the exact neurobiological mechanisms underlying AD pathology may help to provide a biomarker for early diagnosis or at least for assessment of vulnerability to dementia development. Neural plasticity is defined as a capability of the brain to respond to alterations including aging, injury, or learning, with a crucial role of synaptic elements. Long-term potentiation (LTP) and long-term depression (LTD) are important in regulating synaptic connections between neural cells in functional plasticity. Synaptic loss and impairment of the brain's plasticity in AD leads to cognitive impairment, and one of important roles of synaptic biomarkers is monitoring synaptic dysfunction, response to treatment, and predicting future development of AD. Synaptic biomarkers are undoubtedly very promising in developing novel approach to AD treatment and control, especially in the era of aging of societies, which is one of the most common risk factor of AD. Implementing a widespread measurement of synaptic biomarkers of AD will probably be crucial in early diagnosis of AD, early therapeutic intervention, monitoring progression of the disease, or response to treatment. One of the most important challenges is finding a biomarker whose blood concentration correlates with its level in the central nervous system (CNS). This review aims to present the current status of biomarkers of activity-dependent plasticity and persistent enhancement of synaptic transmission in Alzheimer disease.

Chiral Amino Acid Profiling in Serum Reveals Potential Biomarkers for Alzheimer's Disease

BACKGROUND

Alzheimer's disease (AD) is a complex neurodegenerative disease, and increasing evidence has linked dysregulation of amino acids to AD pathogenesis. However, the existing studies often ignore the chirality of amino acids, and some results are inconsistent and controversial. The changes of amino acid profiles in AD from the perspective of enantiomers remain elusive.

OBJECTIVE

The purpose of this study is to investigate whether the levels of amino acids, especially D-amino acids, are deregulated in the peripheral serum of AD patients, with the ultimate goal of discovering novel biomarkers for AD.

METHODS

The chiral amino acid profiles were determined by HPLC-MS/MS with a pre-column derivatization method. Experimental data obtained from 37 AD patients and 34 healthy controls (HC) were statistically analyzed.

RESULTS

Among the 35 amino acids detected, D-proline, D/total-proline ratio, D-aspartate, and D/total-aspartate ratio were decreased, while D-phenylalanine was elevated in AD compared to HC. Significant age-dependent increases in D-proline, D/total-proline ratio, and D-phenylalanine were observed in HC, but not in AD. Receiver operator characteristic analyses of the combination of D-proline, D-aspartate, D-phenylalanine, and age for discriminating AD from HC provided satisfactory area under the curve (0.87), specificity (97.0%), and sensitivity (83.8%). Furthermore, the D-aspartate level was significantly decreased with the progression of AD, as assessed by the Clinical Dementia Rating Scale and Mini-Mental State Examination.

CONCLUSION

The panels of D-proline, D-phenylalanine, and D-aspartate in peripheral serum may serve as novel biomarker candidates for AD. The latter parameter is further associated with the severity of AD.

Protein Expression of Amino Acid Transporters Is Altered in Isolated Cerebral Microvessels of 5xFAD Mouse Model of Alzheimer's Disease

Membrane transporters such as ATP-binding cassette (ABC) and solute carrier (SLC) transporters expressed at the neurovascular unit (NVU) play an important role in drug delivery to the brain and have been demonstrated to be involved in Alzheimer's disease (AD) pathogenesis. However, our knowledge of quantitative changes in transporter absolute protein expression and functionality in vivo in NVU in AD patients and animal models is limited. The study aim was to investigate alterations in protein expression of ABC and SLC transporters in the isolated brain microvessels and brain prefrontal cortices of a widely used model of familial AD, 5xFAD mice (8 months old), using a sensitive liquid chromatography tandem mass spectrometry-based quantitative targeted absolute proteomic approach. Moreover, we examined alterations in brain prefrontal cortical and plasmatic levels of transporter substrates in 5xFAD mice compared to age-matched wild-type (WT) controls. ASCT1 (encoded by Slc1a4) protein expression in the isolated brain microvessels and brain prefrontal cortices of 5xFAD mice was twice higher compared to WT controls (p = 0.01). Brain cortical levels of ASCT1 substrate, serine, were increased in 5xFAD mice compared to WT animals. LAT1 (encoded by Slc7a5) and 4F2hc (encoded by Slc3a2) protein expressions were significantly altered in the isolated brain microvessels of 5xFAD mice compared to WT controls (p = 0.008 and p = 0.05, respectively). Overall, the study provides important information, which is crucial for the optimal use of the 5xFAD mouse model in AD drug development and for investigating novel drug delivery approaches. In addition, the findings of the study shed light on the novel potential mechanisms underlying AD pathogenesis.

Chiral derivatization-enabled discrimination and on-tissue detection of proteinogenic amino acids by ion mobility mass spectrometry

The importance of chiral amino acids (AAs) in living organisms has been widely recognized since the discovery of endogenous d-AAs as potential biomarkers in several metabolic disorders. Chiral analysis by ion mobility spectrometry-mass spectrometry (IMS-MS) has the advantages of high speed and sensitivity but is still in its infancy. Here, an N α-(2,4-dinitro-5-fluorophenyl)-l-alaninamide (FDAA) derivatization is combined with trapped ion mobility spectrometry-mass spectrometry (TIMS-MS) for chiral AA analysis. For the first time, we demonstrate the simultaneous separation of 19 pairs of chiral proteinogenic AAs in a single fixed condition TIMS-MS run. The utility of this approach is presented for mouse brain extracts by direct-infusion TIMS-MS. The robust separation ability in complex biological samples was proven in matrix-assisted laser desorption/ionization (MALDI) TIMS mass spectrometry imaging (MSI) as well by directly depositing 19 pairs of chiral AAs on a tissue slide following on-tissue derivatization. In addition, endogenous chiral amino acids were also detected and distinguished. The developed methods show compelling application prospects in biomarker discovery and biological research.

Assessment of the neuroprotective potential of d-cycloserine and l-serine in aluminum chloride-induced experimental models of Alzheimer’s disease: In vivo and in vitro studies

Alzheimer’s disease (AD) is a neurodegenerative disease characterized by the accumulation of amyloid-β (Aβ) plaques and neurofibrillary tangles in the brain accompanied by synaptic dysfunction and neurodegeneration. No effective treatment has been found to slow the progression of the disease. Therapeutic studies using experimental animal models have therefore become very important. Therefore, this study aimed to investigate the possible neuroprotective effect of D-cycloserine and L-serine against aluminum chloride (AlCl₃)-induced AD in rats. Administration of AlCl₃ for 28 days caused oxidative stress and neurodegeneration compared to the control group. In addition, we found that aluminum decreases α-secretase activity while increasing β-secretase and γ-secretase activities by molecular genetic analysis. D-cycloserine and L-serine application resulted in an improvement in neurodegeneration and oxidative damage caused by aluminum toxicity. It is believed that the results of this study will contribute to the synthesis of new compounds with improved potential against AlCl₃-induced neurodegeneration, cognitive impairment, and drug development research.

Alterations of serine racemase expression determine proliferation and differentiation of neuroblastoma cells

Although the role of serine racemase (SR) in neuropsychiatric disorders has been extensively studied, its role in cell proliferation and differentiation remains unclear. Deletion of Srr, the encoding gene for SR, has been shown to reduce dendritic arborization and dendritic spine density in the brains of adult mice, whereas increased SR levels have been associated with differentiation in cell cultures. Previously, we demonstrated that valproic acid induces differentiation in the N2A neuroblastoma cell line, and that this differentiation is associated with increased SR expression. These observations suggest that SR may have a role in cell proliferation and differentiation. We herein found that both valproic acid and all-trans retinoic acid induced N2A differentiation. In contrast, knockdown of SR reduced levels of differentiation, increased N2A proliferation, promoted cell cycle entry, and modulated expression of cell cycle-related proteins. To further evaluate the effects of SR expression on cell proliferation and differentiation, we used an in vivo model of neuroblastoma in nude mice. N2A cells stably expressing scramble shRNA (Srr^wt -N2A) or specific Srr shRNA (Srr^kd -N2A) were subcutaneously injected into nude mice. The weights and volumes of Srr^wt -N2A-derived tumors were lower than Srr^kd -N2A-derived tumors. Furthermore, Srr^wt -N2A-derived tumors were significantly mitigated by intraperitoneal injection of valproic acid, whereas Srr^kd -N2A-derived tumors were unaffected. Taken together, our findings demonstrate for the first time that alterations in SR expression determine the transition between proliferation and differentiation in neural progenitor cells. Thus, in addition to its well-established roles in neuropsychiatric disorders, our study has highlighted a novel role for SR in cell proliferation and differentiation.

Astrocyte energy and neurotransmitter metabolism in Alzheimer's disease: integration of the glutamate/GABA-glutamine cycle

Astrocytes contribute to the complex cellular pathology of Alzheimer's disease (AD). Neurons and astrocytes function in close collaboration through neurotransmitter recycling, collectively known as the glutamate/GABA-glutamine cycle, which is essential to sustain neurotransmission. Neurotransmitter recycling is intimately linked to astrocyte energy metabolism. In the course of AD, astrocytes undergo extensive metabolic remodeling, which may profoundly affect the glutamate/GABA-glutamine cycle. The consequences of altered astrocyte function and metabolism in relation to neurotransmitter recycling are yet to be comprehended. Metabolic alterations of astrocytes in AD deprive neurons of metabolic support, thereby contributing to synaptic dysfunction and neurodegeneration. In addition, several astrocyte-specific components of the glutamate/GABA-glutamine cycle, including glutamine synthesis and synaptic neurotransmitter uptake, are perturbed in AD. Integration of the complex astrocyte biology within the context of AD is essential for understanding the fundamental mechanisms of the disease, while restoring astrocyte metabolism may serve as an approach to arrest or even revert clinical progression of AD.

Deletion of serine racemase reverses neuronal insulin signaling inhibition by amyloid-β oligomers

Dysregulation of insulin signaling in the Alzheimer's (AD) brain has been extensively reported. Serine racemase(SR) modulates insulin secretion in pancreatic islets. Similarly, we wonder whether or not SR regulates insulin synthesis and secretion in neurons, thereby modulating insulin signaling in the AD brain. Srr-knockout (Srr^-/- ) mice generated with the CRISPR/Cas9 technique were used. Using immunofluorescence and fluorescence in situ hybridization, the levels of insulin protein and insulin(ins2) mRNA significantly increased in the hippocampal but not in the hypothalamic sections of Srr^-/- mice compared with WT mice. Using real-time quantitative PCR, ins2 mRNA from primary hippocampal neuronal cultures of Srr^-/- mice significantly increased compared with the cultured neurons from WT mice. Notably, the secretion of proinsulin C-peptide increased in Srr^-/- neurons relative to WT neurons. By examining the membrane fractional proteins with immunoblotting, Srr^-/- neurons retained ATP-dependent potassium channel on plasmalemma and correspondingly contained higher levels of p-AMPK. Under treatment by Aβ42, the phosphorylation levels of insulin receptor substrate at serine 616,636 (p-IRS1^ser616,636 ) were significantly lower whereas p-AKT³⁰⁸ and p-AKT⁴⁷³ were higher in Srr^-/- neurons, compared with WT neurons, respectively. The phosphorylated form of c-Jun N-terminal kinase decreased in the cultured Srr^-/- neurons relative to the WT neurons upon Aβ42 treatment. In contrast, the phosphorylated protein kinase R remained at the same levels. Further, reactive oxygen species reduced in the cultured Srr^-/- neurons under Aβ42 treatment relative to the WT neurons. Altogether, our study indicated that Srr deletion promoted insulin synthesis and secretion of proinsulin C-peptide, thereby reversing insulin resistance by Aβ42. This study suggests that targeting the neuronal SR may be utilized to enhance insulin signaling which is inhibited at the early stage of the AD brain.

Serum Serine and the Risk of All-Cause Mortality: A Nested Case-Control Study From the China Stroke Primary Prevention Trial (CSPPT)

Background

Serine plays a key role in numerous cellular processes, the levels and metabolism is therefore of critical importance. However, few data are available to illustrate the association of serine with long-term health effects, especially, the predictive value for long-term mortality.

Objective

This study was conducted to evaluate the relationship between serum serine levels and all-cause mortality in general hypertensive patients in a longitudinal cohort, and to examine the potential effect modifiers.

Methods

A nested case-control (NCC) study was conducted utilizing 20702 hypertensive participants from the China Stroke Primary Prevention Trial (CSPPT), a randomized, double-blind, actively controlled trial conducted from May 2008 to August 2013 in China. The current study included 291 cases of all-cause mortality and 291 controls matched on age (≤ 1 year), sex and treatment group. All-cause mortality was the main outcome in this analysis, which included death due to any reason.

Results

With the increase in serum serine levels, the risk of all-cause mortality first increased before flattening. After adjusting for related variables, the risk of mortality increased significantly with the increase of serum serine levels. Compared with group Q1, the mortality risk of group Q2, Q3 and Q4 were significantly increased [ORs, 95% CI: Q2: 2.32, (1.32–4.07); Q3: 2.59, (1.48–4.54); and Q4: 1.85, (1.07–3.22)]. In the exploratory analysis, we observed three effect modifiers, total homocysteine, 5-Methyltetrahydrofolate, and estimated glomerular filtration rate significantly modified the serum serine and all-cause mortality association.

Conclusion

Serum serine levels were significantly associated with an increased risk of all-cause mortality in hypertensive patients. Our results and findings, if confirmed further, suggest that serum serine should be considered as a marker for screening risk factors of mortality.

Clinical Trial Registration

[https://www.clinicaltrials.gov/ct2/show/study/NCT00794885.], identifier [CSPPT, NCT00794885].

Bifunctional super-hydrophilic mesoporous nanocomposite: a novel nanoprobe for investigation of glycosylation and phosphorylation in Alzheimer's disease

Alzheimer's disease (AD) is a common neurodegenerative disease. Abnormal glycosylation and phosphorylation modification in AD may be closely related to its pathology. It is of substantial practical significance to simultaneously investigate the roles of phosphorylation and glycosylation in AD. In this work, a bifunctional super-hydrophilic mesoporous nanocomposite (denoted mTiO₂@AuCG) was prepared, which combined hydrophilic interaction chromatography (HILIC) and metal oxide affinity chromatography (MOAC) enrichment strategies to enrich phosphopeptides and glycopeptides, respectively or simultaneously. The mTiO₂@AuCG exhibited excellent performance on the high-efficiency enrichment of glycopeptides (selectivity, 5000:1 molar ratios of BSA/HRP; sensitivity, 0.1 fmol HRP; satisfactory recovery rate; loading capacity, 200 mg/g) and phosphopeptides (selectivity, 1000:1 molar ratios of BSA/β-casein; sensitivity, 0.2 fmol β-casein; satisfactory recovery rate; loading capacity, 200 mg/g). Using these advantages, after single-step enrichment of mTiO₂@AuCG, a total of 209 glycopeptides related to 93 glycoproteins, and 17 phosphopeptides related to 13 phosphoproteins were detected from normal human serum. By contrast, 167 glycopeptides related to 88 glycoproteins, and 14 phosphopeptides related to 12 phosphoproteins were found in AD serum.

Serine Racemase Expression Differentiates Aging from Alzheimer's Brain

Aging is an inevitable process characterized by progressive loss of physiological integrity and increased susceptibility to cancer, diabetes, cardiovascular, and neurodegenerative diseases; aging is the primary risk factor for Alzheimer's disease (AD), the most common cause of dementia. AD is characterized by brain pathology, including extracellular deposition of amyloid aggregation and intracellular accumulation of neurofibrillary tangles composed of hyperphosphorylated tau protein. In addition, losses of synapses and a wide range of neurons are pivotal pathologies in the AD brain. Accumulating evidence demonstrates hypoactivation of hippocampal neural networks in the aging brain, whereas AD-related mild cognitive impairment (AD-MCI) begins with hyperactivation, followed by a diminution of hippocampal activity as AD develops. The biphasic trends of the activity of the hippocampal neural network are consistent with the alteration of N-methyl-D-aspartate receptor (NMDA-R) activity from aging to prodromal (AD-MCI) to mid-/late stage AD. D-serine, a product of racemization catalyzed by serine racemase (SR), is an important co-agonist of the NMDA-R which is involved in synaptic events including neurotransmission, synaptogenesis, long-term potentiation (LTP), development, and excitotoxicity. SR and D-serine are decreased in the hippocampus of the aging brain, correlating with impairment of cognitive function. By contrast, SR is increased in AD brain, which is associated with a greater degree of cognitive dysfunction. Emerging studies suggest that D-serine levels in the brain or in cerebral spinal fluid from AD patients are higher than in age-matched controls, but the results are inconsistent. Very recently, serum D-serine levels in AD were reported to correlate with sex and clinical dementia rating (CDR) stage. This review will discuss alterations of NMDA-R and SR in aging and AD brain, and the mechanisms underlying the differential regulation of SR will be probed. Collectively, we propose that SR may be a molecular switch that distinguishes the effects of aging from those of AD on the brain.

Complex Processes Underlying the Dynamic Changes of D-serine Levels in AD Brains

BACKGROUND

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by extracellular β-amyloid (Aβ) plaques and cognitive impairments. D-Serine, produced by the enzyme serine racemase (SR) in the brain, functions as an endogenous co-agonist at the glycine-binding site of N-methyl-D-aspartate receptor (NMDAR), has been implicated in the pathophysiological progression of AD.

OBJECTIVES

Evidence regarding the understanding of the role and dynamic modulation of D-serine during AD progression remains controversial. This literature review aims to offer novel research directions for studying the functions and metabolisms of D-serine in AD brains.

METHODS

We searched PubMed, using D-serine/SR and AD as keywords. Studies related to NMDAR dysfunction, neuronal excitotoxicity, D-serine dynamic changes and inflammatory response were included.

RESULTS

This review primarily discusses: (i) Aβ oligomers' role in NMDAR dysregulation, and the subsequent synaptic dysfunction and neuronal damage in AD, (ii) D-serine's role in NMDAR-elicited excitotoxicity, and (iii) the involvement of D-serine and SR in AD-related inflammatory pathological progression.

CONCLUSION

We also presented supposed metabolism and dynamic changes of D-serine during AD progression and hypothesized that: (i) the possible modulation of D-serine levels or SR expression as an effective method of alleviating neurotoxicity during AD pathophysiological progression, and (ii) the dynamic changes of D-serine levels in AD brains possibly resulting from complex processes.

Simultaneous Discrimination and Quantification of Enantiomeric Amino Acids under Physiological Conditions by Chiral 19F NMR Tag

Enantiomeric analysis is of great significance in chemistry, chemical biology and pharmaceutical research. We herein propose a chiral ¹⁹F NMR tag containing an amino reactive NHS group to discriminate the enantiomeric amino acids under physiological conditions by NMR spectroscopy. The chiral ¹⁹F NMR tag readily forms stable amide products with the amino acids in aqueous solution. The stereospecific chemistry of enantiomeric amino acids is discriminated by a stereogenic carbon bonded with a ¹⁹F atom and is therefore decoded by the ¹⁹F reporter and manifested in the distinct ¹⁹F chemical shift. The chemical shift difference (ΔΔδ) of the chiral ¹⁹F NMR tag derived enantiomeric amino acids variants has a broad chemical shift range between -1.13 to 1.68 ppm, indicating the high sensitivity of the chiral ¹⁹F NMR tag to the stereospecific environment surrounding the individual amino acids. The distinguishable chemical shift produced by the chiral ¹⁹F NMR tag permits simultaneous discrimination and quantification of the enantiomeric amino acids in a mixture. The high fidelity of the chiral ¹⁹F NMR tag affords high-accuracy determination of the enantiomeric composition of amino acids by simple 1D NMR under physiological conditions.

Is the Brain Undernourished in Alzheimer's Disease?

Cerebrospinal fluid (CSF) amino acid (AA) levels and CSF/plasma AA ratios in Alzheimer Disease (AD) in relation to nutritional state are not known. Methods: In 30 fasting patients with AD (46% males, 74.4 ± 8.2 years; 3.4 ± 3.2 years from diagnosis) and nine control (CTRL) matched subjects, CSF and venous blood samples were drawn for AA measurements. Patients were stratified according to nutritional state (Mini Nutritional Assessment, MNA, scores). Results: Total CSF/plasma AA ratios were lower in the AD subpopulations than in NON-AD (p < 0.003 to 0.017. In combined malnourished (16.7%; MNA < 17) and at risk for malnutrition (36.6%, MNA 17−24) groups (CG), compared to CTRL, all essential amino acids (EAAs) and 30% of non-EAAs were lower (p < 0.018 to 0.0001), whereas in normo-nourished ADs (46.7%, MNA > 24) the CSF levels of 10% of EAAs and 25% of NON-EAAs were decreased (p < 0.05 to 0.00021). CG compared to normo-nourished ADs, had lower CSF aspartic acid, glutamic acid and Branched-Chain AA levels (all, p < 0.05 to 0.003). CSF/plasma AA ratios were <1 in NON-AD but even lower in the AD population. Conclusions: Compared to CTRL, ADs had decreased CSF AA Levels and CSF/plasma AA ratios, the degree of which depended on nutritional state.

Blood D-Amino Acid Oxidase Levels Increased With Cognitive Decline Among People With Mild Cognitive Impairment: A Two-Year Prospective Study

BACKGROUND

Dysregulation of N-methyl-D-aspartate receptor (NMDAR) neurotransmission has been reported to be implicated in the pathogenesis of Alzheimer's disease (AD). D-amino acid oxidase (DAO), responsible for degradation of NMDAR-related D-amino acids such as D-serine, regulates NMDAR function. A cross-section study found that serum DAO levels were positively related with the severity of cognitive aging among elderly individuals. This 2-year prospective study aimed to explore the role of DAO levels in predicting the outcome of patients with very early-phase AD, such as mild cognitive impairment (MCI).

METHODS

Fifty-one patients with MCI and 21 healthy individuals were recruited. Serum DAO levels and cognitive function, measured by the AD assessment scale-cognitive subscale and the Mini-Mental Status Examination, were monitored every 6 months. We employed multiple regressions to examine the role of DAO concentration in cognitive decline in the 2-year period.

RESULTS

From baseline to endpoint (24 months), serum DAO levels increased significantly, and cognitive ability declined according to both cognitive tests in the MCI patients. Among the healthy individuals, DAO concentrations also increased and Mini-Mental Status Examination scores declined; however, AD assessment scale-cognitive subscale scores did not significantly change. Further, DAO levels at both months 12 and 18 were predictive of cognitive impairment at month 24 among the MCI patients.

CONCLUSIONS

To our knowledge, this is the first study to demonstrate that blood DAO levels increased with cognitive deterioration among the MCI patients in a prospective manner. If replicated by future studies, blood DAO concentration may be regarded as a biomarker for monitoring cognitive change in the patients with MCI.