Serine Racemase Expression Differentiates Aging from Alzheimer's Brain

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.

Design, synthesis, and characterization of novel system xC- transport inhibitors: inhibition of microglial glutamate release and neurotoxicity

Neuroinflammation appears to involve some degree of excitotoxicity promulgated by microglia, which release glutamate via the system xC- (SxC-) cystine-glutamate antiporter. With the aim of mitigating this source of neuronal stress and toxicity, we have developed a panel of inhibitors of the SxC- antiporter. The compounds were based on L-tyrosine, as elements of its structure align with those of glutamate, a primary physiological substrate of the SxC- antiporter. In addition to 3,5-dibromotyrosine, ten compounds were synthesized via amidation of that parent molecule with a selection of acyl halides. These agents were tested for the ability to inhibit release of glutamate from microglia activated with lipopolysaccharide (LPS), an activity exhibited by eight of the compounds. To confirm that the compounds were inhibitors of SxC-, two of them were further tested for the ability to inhibit cystine uptake. Finally, these agents were shown to protect primary cortical neurons from the toxicity exhibited by activated microglia. These agents may hold promise in reducing the neurodegenerative effects of neuroinflammation in conditions, such as encephalitis, traumatic brain injury, stroke, or neurodegenerative diseases.

Transcriptomic Analysis of the Effect of Torin-2 on the Central Nervous System of Drosophila melanogaster

Torin-2, a synthetic compound, is a highly selective inhibitor of both TORC1 and TORC2 (target of rapamycin) complexes as an alternative to the well-known immunosuppressor, geroprotector, and potential anti-cancer natural compound rapamycin. Torin-2 is effective at hundreds of times lower concentrations and prevents some negative side effects of rapamycin. Moreover, it inhibits the rapamycin-resistant TORC2 complex. In this work, we evaluated transcriptomic changes in D. melanogaster heads induced with lifetime diets containing Torin-2 and suggested possible neuroprotective mechanisms of Torin-2. The analysis included D. melanogaster of three ages (2, 4, and 6 weeks old), separately for males and females. Torin-2, taken at the lowest concentration being tested (0.5 μM per 1 L of nutrient paste), had a slight positive effect on the lifespan of D. melanogaster males (+4% on the average) and no positive effect on females. At the same time, RNA-Seq analysis revealed interesting and previously undiscussed effects of Torin-2, which differed between sexes as well as in flies of different ages. Among the cellular pathways mostly altered by Torin-2 at the gene expression level, we identified immune response, protein folding (heat shock proteins), histone modification, actin cytoskeleton organization, phototransduction and sexual behavior. Additionally, we revealed that Torin-2 predominantly reduced the expression of Srr gene responsible for the conversion of L-serine to D-serine and thus regulating activity of NMDA receptor. Via western blot analysis, we showed than in old males Torin-2 tends to increase the ratio of the active phosphorylated form of ERK, the lowest node of the MAPK cascade, which may play a significant role in neuroprotection. Thus, the complex effect of Torin-2 may be due to the interplay of the immune system, hormonal background, and metabolism. Our work is of interest for further research in the field of NMDA-mediated neurodegeneration.

Design, synthesis, and characterization of novel Xc- transport inhibitors: Inhibition of microglial glutamate release and neurotoxicity

Neuroinflammation appears to involve some degree of excitotoxicity promulgated by microglia, which release glutamate via the system Xc- cystine-glutamate antiporter. With the aim of mitigating this source of neuronal stress and toxicity, we have developed a panel of inhibitors of the Xc- antiporter. The compounds were based on L-tyrosine, as elements of its structure align with those of glutamate, a primary physiological substrate of the Xc- antiporter. In addition to 3,5-dibromotyrosine, ten compounds were synthesized via amidation of that parent molecule with a selection of acyl halides. These agents were tested for the ability to inhibit release of glutamate from microglia activated with lipopolysaccharide (LPS), an activity exhibited by eight of the compounds. Two of these were further tested for the ability to inhibit death of primary cortical neurons in the presence of activated microglia. While both showed some neuroprotective activity, they were quantitatively distinct with a compound we refer to as "35DBTA7" showing the greatest effi cacy. This agent may hold promise in reducing the neurodegenerative effects of neuroinflammation in conditions such as encephalitis, traumatic brain injury, stroke, or neurodegenerative diseases.

ASC Transporters Mediate D-Serine Transport into Astrocytes Adjacent to Synapses in the Mouse Brain

D-serine is an important signalling molecule, which activates N-methyl D-aspartate receptors (NMDARs) in conjunction with its fellow co-agonist, the neurotransmitter glutamate. Despite its involvement in plasticity and memory related to excitatory synapses, its cellular source and sink remain a question. We hypothesise that astrocytes, a type of glial cell that surrounds synapses, are likely candidates to control the extracellular concentration of D-Serine by removing it from the synaptic space. Using in situ patch clamp recordings and pharmacological manipulation of astrocytes in the CA1 region of the mouse hippocampal brain slices, we investigated the transport of D-serine across the plasma membrane. We observed the D-serine-induced transport-associated currents upon puff-application of 10 mM D-serine on astrocytes. Further, O-benzyl-L-serine and trans-4-hydroxy-proline, known substrate inhibitors of the alanine serine cysteine transporters (ASCT), reduced D-serine uptake. These results indicate that ASCT is a central mediator of astrocytic D-serine transport and plays a role in regulating its synaptic concentration by sequestration into astrocytes. Similar results were observed in astrocytes of the somatosensory cortex and Bergmann glia in the cerebellum, indicative of a general mechanism expressed across a range of brain areas. This removal of synaptic D-serine and its subsequent metabolic degradation are expected to reduce its extracellular availability, influencing NMDAR activation and NMDAR-dependent synaptic plasticity.

MOLECULAR MECHANISMS UNDERLYING THERAPEUTIC ACTION OF VITAMIN B6

The aim of the study was to analyze the molecular mechanisms that determine the possibility of using vitamin B6 in clinical practice for the correction of various pathological conditions.

Materials and methods. Information retrieval (Scopus, PubMed) and library (eLibrary) databases were used as research tools. In some cases, the ResearchGate application was used for a semantic search. The analysis and generalization of the scientific literature on the topic of research, covering the period from 1989 to the present, has been carried out in the work.

Results. It has been shown that all chemical forms of vitamin B6 are able to penetrate the membranes of most cells by free diffusion, while forming phosphorylated forms inside. Pyridoxal phosphate is a biologically important metabolite that is directly involved as a cofactor in a variety of intracellular reactions. Requirements for this cofactor depend on the age, sex and condition of the patient. Pregnancy and lactation play a special role in the consumption of vitamin B6. In most cases, a balanced diet will provide an acceptable level of this vitamin. At the same time, its deficiency leads to the development of a number of pathological conditions, including neurodegenerative diseases, inflammations and diabetes. Negative manifestations from the central nervous system are also possible with an excessive consumption of B6.

Conclusion. Replenishment of the vitamin B6 level in case of its identified deficiency is a necessary condition for the successful treatment of the central nervous system diseases, diabetes and correction of patients’ immune status. At the same time, it is necessary to observe a balanced intake of this cofactor in order to avoid negative effects on metabolism in case of its excess.

Improved NMDA Receptor Activation by the Secreted Amyloid-Protein Precursor-α in Healthy Aging: A Role for D-Serine?

Impaired activation of the N-methyl-D-aspartate subtype of glutamate receptors (NMDAR) by D-serine is linked to cognitive aging. Whether this deregulation may be used to initiate pharmacological strategies has yet to be considered. To this end, we performed electrophysiological extracellular recordings at CA3/CA1 synapses in hippocampal slices from young and aged mice. We show that 0.1 nM of the soluble N-terminal recombinant fragment of the secreted amyloid-protein precursor-α (sAPPα) added in the bath significantly increased NMDAR activation in aged but not adult mice without impacting basal synaptic transmission. In addition, sAPPα rescued the age-related deficit of theta-burst-induced long-term potentiation. Significant NMDAR improvement occurred in adult mice when sAPPα was raised to 1 nM, and this effect was drastically reduced in transgenic mice deprived of D-serine through genetic deletion of the synthesizing enzyme serine racemase. Altogether, these results emphasize the interest to consider sAPPα treatment targeting D-serine-dependent NMDAR deregulation to alleviate cognitive aging.