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Astrocytes are essential players in development and functions, being particularly relevant as regulators of brain energy metabolism, ionic homeostasis and synaptic transmission. They are also the major source of l‐serine in the brain, which is synthesized from the glycolytic intermediate 3‐phosphoglycerate through the phosphorylated pathway. l‐Serine is the precursor of the two main co‐agonists of the N‐methyl‐d‐aspartate receptor, glycine and d‐serine. Strikingly, dysfunctions in both l‐ and d‐serine metabolism are associated with neurological and psychiatric disorders. Here, we exploited a differentiation protocol, based on the generation of human mature astrocytes from neural stem cells, and investigated the modification of the proteomic and metabolomic profile during the differentiation process. We show that differentiated astrocytes are more similar to mature rather than to reactive ones, and that axogenesis and pyrimidine metabolism increase up to 30 days along with the folate cycle and sphingolipid metabolism. Consistent with the proliferation and cellular maturation processes that are taking place, also the intracellular levels of l‐serine, glycine, threonine, l‐ and d‐aspartate (which level is unexpectedly higher than that of d‐serine) show the same biosynthetic time course. A significant utilization of l‐serine from the medium is apparent while glycine is first consumed and then released with a peak at 30 days, parallel to its intracellular level. These results underline how metabolism changes during astrocyte differentiation, highlight that d‐serine synthesis is restricted in differentiated astrocytes and provide a valuable model for developing potential novel therapeutic approaches to address brain diseases, especially the ones related to serine metabolism alterations. [ABSTRACT FROM AUTHOR]

De novol‐serine biosynthesis in the mammalian astrocytes proceeds via a linear, three‐step pathway (the phosphorylated pathway) catalysed by 3‐phosphoglycerate dehydrogenase (PHGDH), phosphoserine aminotransferase (PSAT) and phosphoserine phosphatase (PSP). The first reaction, catalysed by PHGDH and using the glycolytic intermediate 3‐phosphoglycerate, is strongly shifted towards the reagents, and coupling to the following step by PSAT is required to push the equilibrium towards l‐serine formation; the last step, catalysed by PSP, is virtually irreversible and inhibited by the final product l‐serine. Very little is known about the regulation of the human phosphorylated pathway and the ability of the three enzymes to organise in a complex with potential regulatory functions. Here, the complex formation was investigated in differentiated human astrocytes, by proximity ligation assay, and in vitro on the human recombinant enzymes. The results indicate that the three enzymes co‐localise in cytoplasmic clusters that more stably engage PSAT and PSP. Although in vitro analyses based on native PAGE, size exclusion chromatography and cross‐linking experiments do not show the formation of a stable complex, kinetic studies of the reconstituted pathway using physiological enzyme and substrate concentrations support cluster formation and indicate that PHGDH catalyses the rate‐limiting step while PSP reaction is the driving force for the whole pathway. The enzyme agglomerate assembly of the phosphorylated pathway (the putative 'serinosome') delivers a relevant level of sophistication to the control of l‐serine biosynthesis in human cells, a process strictly related to the modulation of the brain levels of d‐serine and glycine, the main co‐agonists of N‐methyl‐d‐aspartate receptors and various pathological states. [ABSTRACT FROM AUTHOR]

Alzheimer's disease (AD), the main cause of dementia worldwide, is characterized by a complex and multifactorial etiology. In large part, excitatory neurotransmission in the central nervous system is mediated by glutamate and its receptors are involved in synaptic plasticity. The N-methyl-D-aspartate (NMDA) receptors, which require the agonist glutamate and a coagonist such as glycine or the D-enantiomer of serine for activation, play a main role here. A second D-amino acid, D-aspartate, acts as agonist of NMDA receptors. D-amino acids, present in low amounts in nature and long considered to be of bacterial origin, have distinctive functions in mammals. In recent years, alterations in physiological levels of various D-amino acids have been linked to various pathological states, ranging from chronic kidney disease to neurological disorders. Actually, the level of NMDA receptor signaling must be balanced to promote neuronal survival and prevent neurodegeneration: this signaling in AD is affected mainly by glutamate availability and modulation of the receptor's functions. Here, we report the experimental findings linking D-serine and D-aspartate, through NMDA receptor modulation, to AD and cognitive functions. Interestingly, AD progression has been also associated with the enzymes related to D-amino acid metabolism as well as with glucose and serine metabolism. Furthermore, the D-serine and D-/total serine ratio in serum have been recently proposed as biomarkers of AD progression. A greater understanding of the role of D-amino acids in excitotoxicity related to the pathogenesis of AD will facilitate novel therapeutic treatments to cure the disease and improve life expectancy. [ABSTRACT FROM AUTHOR]

Healthy aging is an ambitious aspiration for humans, but neurodegenerative disorders, such as Alzheimer's disease (AD), strongly affect quality of life. Using an integrated omics approach, we investigate alterations in the molecular composition of postmortem hippocampus samples of healthy persons and individuals with AD. Profound differences are apparent between control and AD male and female cohorts in terms of up- and downregulated metabolic pathways. A decrease in the insulin response is evident in AD when comparing the female with the male group. The serine metabolism (linked to the glycolytic pathway and generating the N-methyl-D-aspartate [NMDA] receptor coagonist D-serine) is also significantly modulated: the D-Ser/total serine ratio represents a way to counteract age-related cognitive decline in healthy men and during AD onset in women. These results show how AD changes and, in certain respects, almost reverses sex-specific proteomic and metabolomic profiles, highlighting how different pathophysiological mechanisms are active in men and women. [Display omitted] • Multi-omics analysis identified sex-specific AD hallmarks in human hippocampus • D-Ser/total Ser ratio increases in the hippocampus of women with AD • The insulin pathway decreases in women compared with men with AD • The alterations in Arg and Ala/Asp/Glu metabolism in AD are not sex specific Maffioli et al. perform a multi-omics analysis of the hippocampus of individuals with Alzheimer's disease (AD) and healthy controls. Considering the sex of the individuals, women with AD display a decrease in the insulin response and an increase in the D-serine/total serine ratio compared with men with AD. [ABSTRACT FROM AUTHOR]

The human enzyme D-3-phosphoglycerate dehydrogenase (hPHGDH) catalyzes the reversible dehydrogenation of 3-phosphoglycerate (3PG) into 3-phosphohydroxypyruvate (PHP) using the NAD+/NADH redox cofactor, the first step in the phosphorylated pathway producing L-serine. We focused on the full-length enzyme that was produced in fairly large amounts in E. coli cells; the effect of pH, temperature and ligands on hPHGDH activity was studied. The forward reaction was investigated on 3PG and alternative carboxylic acids by employing two coupled assays, both removing the product PHP; 3PG was by far the best substrate in the forward direction. Both PHP and α-ketoglutarate were efficiently reduced by hPHGDH and NADH in the reverse direction, indicating substrate competition under physiological conditions. Notably, neither PHP nor L-serine inhibited hPHGDH, nor did glycine and D-serine, the coagonists of NMDA receptors related to L-serine metabolism. The investigation of NADH and phosphate binding highlights the presence in solution of different conformations and/or oligomeric states of the enzyme. Elucidating the biochemical properties of hPHGDH will enable the identification of novel approaches to modulate L-serine levels and thus to reduce cancer progression and treat neurological disorders. [ABSTRACT FROM AUTHOR]