Brain-specific Phgdh deletion reveals a pivotal role for L-serine biosynthesis in controlling the level of D-serine, an N-methyl-D-aspartate receptor co-agonist, in adult brain

Transcriptional regulation by PHGDH drives amyloid pathology in Alzheimer's disease

Virtually all individuals aged 65 or older develop at least early pathology of Alzheimer's disease (AD), yet most lack disease-causing mutations in APP, PSEN, or MAPT, and many do not carry the APOE4 risk allele. This raises questions about AD development in the general population. Although transcriptional dysregulation has not traditionally been a hallmark of AD, recent studies reveal significant epigenomic changes in late-onset AD (LOAD) patients. We show that altered expression of the LOAD biomarker phosphoglycerate dehydrogenase (PHGDH) modulates AD pathology in mice and human brain organoids independent of its enzymatic activity. PHGDH has an uncharacterized role in transcriptional regulation, promoting the transcription of inhibitor of nuclear factor kappa-B kinase subunit alpha (IKKa) and high-mobility group box 1 (HMGB1) in astrocytes, which suppress autophagy and accelerate amyloid pathology. A blood-brain-barrier-permeable small-molecule inhibitor targeting PHGDH's transcriptional function reduces amyloid pathology and improves AD-related behavioral deficits. These findings highlight transcriptional regulation in LOAD and suggest therapeutic strategies beyond targeting familial mutations.

L-serine metabolic regulation and host respiratory homeostasis

L-Serine, a non-essential amino acid (NEAA), can be obtained through diet or in situ synthesis. Functionally, L-serine not only serves as the precursor of other amino acids, lipids, and nucleotides, but also participates in the folate/methionine cycle. An increasing number of studies have demonstrated that L-serine is widely used in the adjuvant therapy of many diseases (e.g., inflammation, infections, fibrosis, and tumors). Here, we summarize the synthesis and metabolism of serine followed by its functions in health and disease. Moreover, we delineate the potential mechanisms whereby L-serine is involved in the occurrence and progression of respiratory diseases. This review aims to summarize the research progress of serine in diseases, propose the problems that need to be solved in the future, and provide guidance for subsequent research and development.

Possible Metabolic Remodeling based on de novo Biosynthesis of L-serine in Se-Subtoxic or -Deficient Mammals

Current research studies point to an increased risk of diabetes with selenium (Se) intake beyond the physiological requirement used to prevent cancers. The existing hypothesis of "selenoprotein overexpression leads to intracellular redox imbalance" cannot clearly explain the U-shaped dose-effect relationship between Se intake and the risk of diabetes. In this review, it is speculated that metabolic remodeling based on the de novo biosynthesis of L-serine may occur in mammals at supranutritional or subtoxic levels of Se. It is also speculated that a large amount of L-serine is consumed by the body during insufficient Se intake, thus resulting in similar metabolic reprogramming. The increase in atypical ceramide and its derivatives due to the lack of L-serine may also play a role in the development of diabetes.

Serine and glycine physiology reversibly modulate retinal and peripheral nerve function

Metabolic homeostasis is maintained by redundant pathways to ensure adequate nutrient supply during fasting and other stresses. These pathways are regulated locally in tissues and systemically via the liver, kidney, and circulation. Here, we characterize how serine, glycine, and one-carbon (SGOC) metabolism fluxes across the eye, liver, and kidney sustain retinal amino acid levels and function. Individuals with macular telangiectasia (MacTel), an age-related retinal disease with reduced circulating serine and glycine, carrying deleterious alleles in SGOC metabolic enzymes exhibit an exaggerated reduction in circulating serine. A Phgdh+/- mouse model of this haploinsufficiency experiences accelerated retinal defects upon dietary serine/glycine restriction, highlighting how otherwise silent haploinsufficiencies can impact retinal health. We demonstrate that serine-associated retinopathy and peripheral neuropathy are reversible, as both are restored in mice upon serine supplementation. These data provide molecular insights into the genetic and metabolic drivers of neuro-retinal dysfunction while highlighting therapeutic opportunities to ameliorate this pathogenesis.

Control of epigenomic landscape and development of fetal male germ cells through L-serine metabolism

Sex-specific metabolic characteristics emerge in the mouse germ line after reaching the genital ridges around embryonic day 10.5, coinciding with sexual differentiation. However, the impact of such metabolic characteristics on germ cell development remains unclear. In this study, we observed the specific upregulation in male fetal germ cells of D-3-phosphoglycerate dehydrogenase (PHGDH), the primary enzyme in the serine-glycine-one-carbon metabolism, along with an increase in a downstream metabolite, S-adenosylmethionine (SAM), crucial for protein and nucleic acid methylation. Inhibiting PHGDH in fetal testes resulted in reduced SAM levels in germ cells, accompanied by increases in the number of mouse vasa homolog (MVH/VASA)-positive germ cells and the promyelocytic leukemia zinc finger (PLZF)-positive undifferentiated spermatogonia ratio. Furthermore, PHGDH inhibition led to a decrease in the methylation of histone H3 and DNA, resulting in aberrations in gene expression profiles. In summary, our findings underscore the significant role of certain metabolic mechanisms in the development of male germ cells.

On the quaternary structure of human D-3-phosphoglycerate dehydrogenase

D-3-phosphoglycerate dehydrogenase (PHGDH) catalyzes the NAD+-dependent conversion of D-3-phospho-glycerate to 3-phosphohydroxypyruvate, the first step in the phosphorylated pathway for L-serine (L-Ser) biosynthesis. L-Ser plays different relevant metabolic roles in eukaryotic cells: alterations in L-Ser metabolism have been linked to serious neurological disorders. The human PHGDH (hPHGDH), showing a homotetrameric state in solution, is made of four domains, among which there are two regulatory domains at the C-terminus: the aspartate kinase-chorismate mutase-tyrA prephenate dehydrogenase (ACT) and allosteric substrate-binding (ASB) domains. The structure of hPHGDH was solved only for a truncated, dimeric form harboring the N-terminal end containing the substrate and the cofactor binding domains. A model ensemble of the tetrameric hPHGDH was generated using AlphaFold coupled with molecular dynamics refinement. By analyzing the inter-subunit interactions at the tetrameric interface, the residues F418, L478, P479, R454, and Y495 were selected and their role was studied by the alanine-scanning mutagenesis approach. The F418A variant modifies the putative ASB, slightly alters the activity, the fraction of protein in the tetrameric state, and the protein stability; it seems relevant in dimers' recognition to yield the tetrameric oligomer. On the contrary, the R454A, L478A, P479A, and Y495A variants (ACT domain) determine a loss of the tetrameric assembly, resulting in low stability and misfolding, triggering the aggregation and hampering the activity. The predicted tetrameric interface seems mediated by residues at the ACT domain, and the tetramer formation seems crucial for proper folding of hPHGDH, which, in turn, is essential for both stability and functionality.

Constitutive activity of ionotropic glutamate receptors via hydrophobic substitutions in the ligand-binding domain

Neurotransmitter ligands electrically excite neurons by activating ionotropic glutamate receptor (iGluR) ion channels. Knowledge of the iGluR amino acid residues that dominate ligand-induced activation would enable the prediction of function from sequence. We therefore explored the molecular determinants of activity in rat N-methyl-D-aspartate (NMDA)-type iGluRs (NMDA receptors), complex heteromeric iGluRs comprising two glycine-binding GluN1 and two glutamate-binding GluN2 subunits, using amino acid sequence analysis, mutagenesis, and electrophysiology. We find that a broadly conserved aspartate residue controls both ligand potency and channel activity, to the extent that certain substitutions at this position bypass the need for ligand binding in GluN1 subunits, generating NMDA receptors activated solely by glutamate. Furthermore, we identify a homomeric iGluR from the placozoan Trichoplax adhaerens that has utilized native mutations of this crucial residue to evolve into a leak channel that is inhibited by neurotransmitter binding, pointing to a dominant role of this residue throughout the iGluR superfamily.

Biochemical and cellular studies of three human 3-phosphoglycerate dehydrogenase variants responsible for pathological reduced L-serine levels

In the brain, the non-essential amino acid L-serine is produced through the phosphorylated pathway (PP) starting from the glycolytic intermediate 3-phosphoglycerate: among the different roles played by this amino acid, it can be converted into D-serine and glycine, the two main co-agonists of NMDA receptors. In humans, the enzymes of the PP, namely phosphoglycerate dehydrogenase (hPHGDH, which catalyzes the first and rate-limiting step of this pathway), 3-phosphoserine aminotransferase, and 3-phosphoserine phosphatase are likely organized in the cytosol as a metabolic assembly (a "serinosome"). The hPHGDH deficiency is a pathological condition biochemically characterized by reduced levels of L-serine in plasma and cerebrospinal fluid and clinically identified by severe neurological impairment. Here, three single-point variants responsible for hPHGDH deficiency and Neu-Laxova syndrome have been studied. Their biochemical characterization shows that V261M, V425M, and V490M substitutions alter either the kinetic (both maximal activity and Km for 3-phosphoglycerate in the physiological direction) and the structural properties (secondary, tertiary, and quaternary structure, favoring aggregation) of hPHGDH. All the three variants have been successfully ectopically expressed in U251 cells, thus the pathological effect is not due to hindered expression level. At the cellular level, mistargeting and aggregation phenomena have been observed in cells transiently expressing the pathological protein variants, as well as a reduced L-serine cellular level. Previous studies demonstrated that the pharmacological supplementation of L-serine in hPHGDH deficiencies could ameliorate some of the related symptoms: our results now suggest the use of additional and alternative therapeutic approaches.

Impairment of serine transport across the blood-brain barrier by deletion of Slc38a5 causes developmental delay and motor dysfunction

Brain L-serine is critical for neurodevelopment and is thought to be synthesized solely from glucose. In contrast, we found that the influx of L-serine across the blood-brain barrier (BBB) is essential for brain development. We identified the endothelial Slc38a5, previously thought to be a glutamine transporter, as an L-serine transporter expressed at the BBB in early postnatal life. Young Slc38a5 knockout (KO) mice exhibit developmental alterations and a decrease in brain L-serine and D-serine, without changes in serum or liver amino acids. Slc38a5-KO brains exhibit accumulation of neurotoxic deoxysphingolipids, synaptic and mitochondrial abnormalities, and decreased neurogenesis at the dentate gyrus. Slc38a5-KO pups exhibit motor impairments that are affected by the administration of L-serine at concentrations that replenish the serine pool in the brain. Our results highlight a critical role of Slc38a5 in supplying L-serine via the BBB for proper brain development.

Bioinformatics Analysis of the Molecular Networks Associated with the Amelioration of Aberrant Gene Expression by a Tyr-Trp Dipeptide in Brains Treated with the Amyloid-β Peptide

Short-chain peptides derived from various protein sources have been shown to exhibit diverse bio-modulatory and health-promoting effects in animal experiments and human trials. We recently reported that the oral administration of the Tyr-Trp (YW) dipeptide to mice markedly enhances noradrenaline metabolism in the brain and ameliorates the working-memory deficits induced by the β-amyloid 25-35 peptide (Aβ_25-35). In the current study, we performed multiple bioinformatics analyses of microarray data from Aβ_25-35/YW-treated brains to determine the mechanism underlying the action of YW in the brain and to infer the molecular mechanisms and networks involved in the protective effect of YW in the brain. We found that YW not only reversed inflammation-related responses but also activated various molecular networks involving a transcriptional regulatory system, which is mediated by the CREB binding protein (CBP), EGR-family proteins, ELK1, and PPAR, and the calcium-signaling pathway, oxidative stress tolerance, and an enzyme involved in de novo l-serine synthesis in brains treated with Aβ_25-35. This study revealed that YW has a neuroprotective effect against Aβ_25-35 neuropathy, suggesting that YW is a new functional-food-material peptide.

Rational and Translational Implications of D-Amino Acids for Treatment-Resistant Schizophrenia: From Neurobiology to the Clinics

Schizophrenia has been conceptualized as a neurodevelopmental disorder with synaptic alterations and aberrant cortical–subcortical connections. Antipsychotics are the mainstay of schizophrenia treatment and nearly all share the common feature of dopamine D2 receptor occupancy, whereas glutamatergic abnormalities are not targeted by the presently available therapies. D-amino acids, acting as N-methyl-D-aspartate receptor (NMDAR) modulators, have emerged in the last few years as a potential augmentation strategy in those cases of schizophrenia that do not respond well to antipsychotics, a condition defined as treatment-resistant schizophrenia (TRS), affecting almost 30–40% of patients, and characterized by serious cognitive deficits and functional impairment. In the present systematic review, we address with a direct and reverse translational perspective the efficacy of D-amino acids, including D-serine, D-aspartate, and D-alanine, in poor responders. The impact of these molecules on the synaptic architecture is also considered in the light of dendritic spine changes reported in schizophrenia and antipsychotics’ effect on postsynaptic density proteins. Moreover, we describe compounds targeting D-amino acid oxidase and D-aspartate oxidase enzymes. Finally, other drugs acting at NMDAR and proxy of D-amino acids function, such as D-cycloserine, sarcosine, and glycine, are considered in the light of the clinical burden of TRS, together with other emerging molecules.

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.

PHGDH expression increases with progression of Alzheimer's disease pathology and symptoms

Chen et al. reveal an increase of phosphoglycerate dehydrogenase (PHGDH) mRNA and protein levels in two mouse models and four human cohorts in Alzheimer's disease brains compared to age- and sex-matched control brains. The increase of PHGDH expression in human brain correlates with symptomatic development and disease pathology.

SLC6A and SLC16A family of transporters: Contribution to transport of creatine and creatine precursors in creatine biosynthesis and distribution

Creatine (Cr) is needed to maintain high energy levels in cells. Since Cr plays reportedly a critical role in neurodevelopment and the immune system, Cr dynamics should be strictly regulated to control these physiological events. This review focuses on the role of transporters that recognize Cr and/or Cr precursors. Our previous studies revealed physiological roles of SLC6A and SLC16A family transporters in Cr dynamics. Creatine transporter (CRT/SLC6A8) contributes to the influx transport of Cr in Cr distribution. γ-Aminobutyric acid transporter 2 (GAT2/SLC6A13) mediates incorporation of guanidinoacetate (GAA), a Cr precursor, in the process of Cr biosynthesis. Monocarboxylate transporter 12 (MCT12/SLC16A12) functions as an efflux transporter for Cr and GAA, and contributes to the process of Cr biosynthesis. Accordingly, the SLC6A and SLC16A family of transporters play important roles in the process of Cr biosynthesis and distribution via permeation of Cr and Cr precursors across the plasma membrane.

Hepatocyte-Specific Phgdh-Deficient Mice Culminate in Mild Obesity, Insulin Resistance, and Enhanced Vulnerability to Protein Starvation

l-Serine (Ser) is synthesized de novo from 3-phosphoglycerate via the phosphorylated pathway committed by phosphoglycerate dehydrogenase (Phgdh). A previous study reported that feeding a protein-free diet increased the enzymatic activity of Phgdh in the liver and enhanced Ser synthesis in the rat liver. However, the nutritional and physiological functions of Ser synthesis in the liver remain unclear. To clarify the physiological significance of de novo Ser synthesis in the liver, we generated liver hepatocyte-specific Phgdh KO (LKO) mice using an albumin-Cre driver. The LKO mice exhibited a significant gain in body weight compared to Floxed controls at 23 weeks of age and impaired systemic glucose metabolism, which was accompanied by diminished insulin/IGF signaling. Although LKO mice had no apparent defects in steatosis, the molecular signatures of inflammation and stress responses were evident in the liver of LKO mice. Moreover, LKO mice were more vulnerable to protein starvation than the Floxed mice. These observations demonstrate that Phgdh-dependent de novo Ser synthesis in liver hepatocytes contributes to the maintenance of systemic glucose tolerance, suppression of inflammatory response, and resistance to protein starvation.

A structural view onto disease-linked mutations in the human neutral amino acid exchanger ASCT1

The ASCT1 transporter of the SLC1 family is largely involved in equilibration of neutral amino acids' pools across the plasma membrane and plays a prominent role in the transport of both L- and D-isomers of serine, essential for the normal functioning of the central nervous system in mammals. A number of mutations in ASCT1 (E256K, G381R, R457W) have been linked to severe neurodevelopmental disorders, however in the absence of ASCT1 structure it is hard to understand their impact on substrate transport. To ameliorate that we have determined a cryo-EM structure of human ASCT1 at 4.2 Å resolution and performed functional transport assays and molecular dynamics simulations, which revealed that given mutations lead to the diminished transport capability of ASCT1 caused by instability of transporter and impeded transport cycle.

INPP5K and SIL1 associated pathologies with overlapping clinical phenotypes converge through dysregulation of PHGDH

Marinesco-Sjögren syndrome is a rare human disorder caused by biallelic mutations in SIL1 characterized by cataracts in infancy, myopathy and ataxia, symptoms which are also associated with a novel disorder caused by mutations in INPP5K. While these phenotypic similarities may suggest commonalties at a molecular level, an overlapping pathomechanism has not been established yet. In this study, we present six new INPP5K patients and expand the current mutational and phenotypical spectrum of the disease showing the clinical overlap between Marinesco-Sjögren syndrome and the INPP5K phenotype. We applied unbiased proteomic profiling on cells derived from Marinesco-Sjögren syndrome and INPP5K patients and identified alterations in d-3-PHGDH as a common molecular feature. d-3-PHGDH modulates the production of l-serine and mutations in this enzyme were previously associated with a neurological phenotype, which clinically overlaps with Marinesco-Sjögren syndrome and INPP5K disease. As l-serine administration represents a promising therapeutic strategy for d-3-PHGDH patients, we tested the effect of l-serine in generated sil1, phgdh and inpp5k a+b zebrafish models, which showed an improvement in their neuronal phenotype. Thus, our study defines a core phenotypical feature underpinning a key common molecular mechanism in three rare diseases and reveals a common and novel therapeutic target for these patients.

Astrocyte-neuron metabolic cooperation shapes brain activity

The brain has almost no energy reserve, but its activity coordinates organismal function, a burden that requires precise coupling between neurotransmission and energy metabolism. Deciphering how the brain accomplishes this complex task is crucial to understand central facets of human physiology and disease mechanisms. Each type of neural cell displays a peculiar metabolic signature, forcing the intercellular exchange of metabolites that serve as both energy precursors and paracrine signals. The paradigm of this biological feature is the astrocyte-neuron couple, in which the glycolytic metabolism of astrocytes contrasts with the mitochondrial oxidative activity of neurons. Astrocytes generate abundant mitochondrial reactive oxygen species and shuttle to neurons glycolytically derived metabolites, such as L-lactate and L-serine, which sustain energy needs, conserve redox status, and modulate neurotransmitter-receptor activity. Conversely, early disruption of this metabolic cooperation may contribute to the initiation or progression of several neurological diseases, thus requiring innovative therapies to preserve brain energetics.

NMDARs, Coincidence Detectors of Astrocytic and Neuronal Activities

Synaptic plasticity is an extensively studied cellular correlate of learning and memory in which NMDARs play a starring role. One of the most interesting features of NMDARs is their ability to act as a co-incident detector. It is unique amongst neurotransmitter receptors in this respect. Co-incident detection is possible because the opening of NMDARs requires membrane depolarisation and the binding of glutamate. Opening of NMDARs also requires a co-agonist. Although the dynamic regulation of glutamate and membrane depolarization have been well studied in coincident detection, the role of the co-agonist site is unexplored. It turns out that non-neuronal glial cells, astrocytes, regulate co-agonist availability, giving them the ability to influence synaptic plasticity. The unique morphology and spatial arrangement of astrocytes at the synaptic level affords them the capacity to sample and integrate information originating from unrelated synapses, regardless of any pre-synaptic and post-synaptic commonality. As astrocytes are classically considered slow responders, their influence at the synapse is widely recognized as modulatory. The aim herein is to reconsider the potential of astrocytes to participate directly in ongoing synaptic NMDAR activity and co-incident detection.

Not Just a Bystander: The Emerging Role of Astrocytes and Research Tools in Studying Cognitive Dysfunctions in Schizophrenia

Cognitive dysfunction is one of the core symptoms in schizophrenia, and it is predictive of functional outcomes and therefore useful for treatment targets. Rather than improving cognitive deficits, currently available antipsychotics mainly focus on positive symptoms, targeting dopaminergic/serotoninergic neurons and receptors in the brain. Apart from investigating the neural mechanisms underlying schizophrenia, emerging evidence indicates the importance of glial cells in brain structure development and their involvement in cognitive functions. Although the etiopathology of astrocytes in schizophrenia remains unclear, accumulated evidence reveals that alterations in gene expression and astrocyte products have been reported in schizophrenic patients. To further investigate the role of astrocytes in schizophrenia, we highlighted recent progress in the investigation of the effect of astrocytes on abnormalities in glutamate transmission and impairments in the blood–brain barrier. Recent advances in animal models and behavioral methods were introduced to examine schizophrenia-related cognitive deficits and negative symptoms. We also highlighted several experimental tools that further elucidate the role of astrocytes. Instead of focusing on schizophrenia as a neuron-specific disorder, an additional astrocytic perspective provides novel and promising insight into its causal mechanisms and treatment. The involvement of astrocytes in the pathogenesis of schizophrenia and other brain disorders is worth further investigation.

Forebrain expression of serine racemase during postnatal development

N-methyl-D-aspartate receptors (NMDARs) are important for synaptogenesis, synaptic maturation and refinement during the early postnatal weeks after birth. Defective synapse formation or refinement underlie cognitive and emotional abnormalities in various neurodevelopmental disorders (NDDs), including schizophrenia (Sz) and autism spectrum disorder (ASD). Serine racemase (SR) is a neuronal enzyme that produces D-serine, a co-agonist required for full NMDAR activation. NMDAR hypofunction as a result of genetic SR elimination and reduced synaptic availability of D-serine reduces neuronal dendritic arborization and spine density. In adult mouse brain, the expression of SR parallels that of NMDARs across forebrain regions including the striatum, amygdala, hippocampus, and medial prefrontal cortex (mPFC). However, there have yet to be studies providing a detailed characterization of the spatial and temporal expression of SR during early periods of synaptogenesis. Here, we examined the postnatal expression of SR in cortical and subcortical brain regions important for learning, memory and emotional regulation, during the first four weeks after birth. Using dual-antigen immunofluorescence, we demonstrate that the number of SR+ neurons steadily increases with postnatal age across the mPFC, amygdala, hippocampus and striatum. We also identified differences in the rate of SR protein induction both across and within brain regions. Analyzing existing human post-mortem brain in situ data, there was a similar developmental mRNA expression profile of SRR and GRIN1 (GluN1 subunit) from infancy through the first decade of life. Our findings further support a developmental role for D-serine mediated NMDAR activation regulating synaptogenesis and neural circuit refinement, which has important implications for the pathophysiology of Sz and other NDDs.

Astrocytes in schizophrenia

Schizophrenia is a severe and clinically heterogenous mental disorder affecting approximately 1% of the population worldwide. Despite tremendous achievements in the field of schizophrenia research, its precise aetiology remains elusive. Besides dysfunctional neuronal signalling, the pathophysiology of schizophrenia appears to involve molecular and functional abnormalities in glial cells, including astrocytes. This article provides a concise overview of the current evidence supporting altered astrocyte activity in schizophrenia, which ranges from findings obtained from post-mortem immunohistochemical analyses, genetic association studies and transcriptomic investigations, as well as from experimental investigations of astrocyte functions in animal models. Integrating the existing data from these research areas strongly suggests that astrocytes have the capacity to critically affect key neurodevelopmental and homeostatic processes pertaining to schizophrenia pathogenesis, including glutamatergic signalling, synaptogenesis, synaptic pruning and myelination. The further elucidation of astrocytes functions in health and disease may, therefore, offer new insights into how these glial cells contribute to abnormal brain development and functioning underlying this debilitating mental disorder.

Biochemical and Biophysical Characterization of Recombinant Human 3-Phosphoglycerate Dehydrogenase

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.

Effect of selectively knocking down key metabolic genes in Müller glia on photoreceptor health

The importance of Müller glia for retinal homeostasis suggests that they may have vulnerabilities that lead to retinal disease. Here, we studied the effect of selectively knocking down key metabolic genes in Müller glia on photoreceptor health. Immunostaining indicated that murine Müller glia expressed insulin receptor (IR), hexokinase 2 (HK2) and phosphoglycerate dehydrogenase (PHGDH) but very little pyruvate dehydrogenase E1 alpha 1 (PDH-E1α) and lactate dehydrogenase A (LDH-A). We crossed Müller glial cell-CreER (MC-CreER) mice with transgenic mice carrying a floxed IR, HK2, PDH-E1α, LDH-A, or PHGDH gene to study the effect of selectively knocking down key metabolic genes in Müller glia cells on retinal health. Selectively knocking down IR, HK2, or PHGDH led to photoreceptor degeneration and reduced electroretinographic responses. Supplementing exogenous l-serine prevented photoreceptor degeneration and improved retinal function in MC-PHGDH knockdown mice. We unexpectedly found that the levels of retinal serine and glycine were not reduced but, on the contrary, highly increased in MC-PHGDH knockdown mice. Moreover, dietary serine supplementation, while rescuing the retinal phenotypes caused by genetic deletion of PHGDH in Müller glial cells, restored retinal serine and glycine homeostasis probably through regulation of serine transport. No retinal abnormalities were observed in MC-CreER mice crossed with PDH-E1α- or LDH-A-floxed mice despite Cre expression. Our findings suggest that Müller glia do not complete glycolysis but use glucose to produce serine to support photoreceptors. Supplementation with exogenous serine is effective in preventing photoreceptor degeneration caused by PHGDH deficiency in Müller glia.

Dimerization of PHGDH via the catalytic unit is essential for its enzymatic function

Human D-3-phosphoglycerate dehydrogenase (PHGDH), a key enzyme in de novo serine biosynthesis, is amplified in various cancers and serves as a potential target for anticancer drug development. To facilitate this process, more information is needed on the basic biochemistry of this enzyme. For example, PHGDH was found to form tetramers in solution and the structure of its catalytic unit (sPHGDH) was solved as a dimer. However, how the oligomeric states affect PHGDH enzyme activity remains elusive. We studied the dependence of PHGDH enzymatic activity on its oligomeric states. We found that sPHGDH forms a mixture of monomers and dimers in solution with a dimer dissociation constant of ∼0.58 μM, with the enzyme activity depending on the dimer content. We computationally identified hotspot residues at the sPHGDH dimer interface. Single-point mutants at these sites disrupt dimer formation and abolish enzyme activity. Molecular dynamics simulations showed that dimer formation facilitates substrate binding and maintains the correct conformation required for enzyme catalysis. We further showed that the full-length PHGDH exists as a dynamic mixture of monomers, dimers, and tetramers in solution with enzyme concentration-dependent activity. Mutations that can completely disrupt the sPHGDH dimer show different abilities to interrupt the full-length PHGDH tetramer. Among them, E108A and I121A can also disrupt the oligomeric structures of the full-length PHGDH and abolish its enzyme activity. Our study indicates that disrupting the oligomeric structure of PHGDH serves as a novel strategy for PHGDH drug design and the hotspot residues identified can guide the design process.

Role of the HSP70 Co-Chaperone SIL1 in Health and Disease

Cell surface and secreted proteins provide essential functions for multicellular life. They enter the endoplasmic reticulum (ER) lumen co-translationally, where they mature and fold into their complex three-dimensional structures. The ER is populated with a host of molecular chaperones, associated co-factors, and enzymes that assist and stabilize folded states. Together, they ensure that nascent proteins mature properly or, if this process fails, target them for degradation. BiP, the ER HSP70 chaperone, interacts with unfolded client proteins in a nucleotide-dependent manner, which is tightly regulated by eight DnaJ-type proteins and two nucleotide exchange factors (NEFs), SIL1 and GRP170. Loss of SIL1′s function is the leading cause of Marinesco-Sjögren syndrome (MSS), an autosomal recessive, multisystem disorder. The development of animal models has provided insights into SIL1′s functions and MSS-associated pathologies. This review provides an in-depth update on the current understanding of the molecular mechanisms underlying SIL1′s NEF activity and its role in maintaining ER homeostasis and normal physiology. A precise understanding of the underlying molecular mechanisms associated with the loss of SIL1 may allow for the development of new pharmacological approaches to treat MSS.

Astroglial cells as neuroendocrine targets in forebrain development: Implications for sex differences in psychiatric disease

Astroglial cells are the most abundant cell type in the mammalian brain. They are implicated in almost every aspect of brain physiology, including maintaining homeostasis, building and maintaining the blood brain barrier, and the development and maturation of neuronal networks. Critically, astroglia also express receptors for gonadal sex hormones, respond rapidly to gonadal hormones, and are able to synthesize hormones. Thus, they are positioned to guide and mediate sexual differentiation of the brain, particularly neuronal networks in typical and pathological conditions. In this review, we describe astroglial involvement in the organization and development of the brain, and consider known sex differences in astroglial responses to understand how astroglial cell-mediated organization may play a role in forebrain sexual dimorphisms in human populations. Finally, we consider how sexually dimorphic astroglial responses and functions in development may lead to sex differences in vulnerability for neuropsychiatric disorders.

L-serine synthesis via the phosphorylated pathway in humans

L-serine is a nonessential amino acid in eukaryotic cells, used for protein synthesis and in producing phosphoglycerides, glycerides, sphingolipids, phosphatidylserine, and methylenetetrahydrofolate. Moreover, L-serine is the precursor of two relevant coagonists of NMDA receptors: glycine (through the enzyme serine hydroxymethyltransferase), which preferentially acts on extrasynaptic receptors and D-serine (through the enzyme serine racemase), dominant at synaptic receptors. The cytosolic "phosphorylated pathway" regulates de novo biosynthesis of L-serine, employing 3-phosphoglycerate generated by glycolysis and the enzymes 3-phosphoglycerate dehydrogenase, phosphoserine aminotransferase, and phosphoserine phosphatase (the latter representing the irreversible step). In the human brain, L-serine is primarily found in glial cells and is supplied to neurons for D-serine synthesis. Serine-deficient patients show severe neurological symptoms, including congenital microcephaly, psychomotor retardation, and intractable seizures, thus highlighting the relevance of de novo production of this amino acid in brain development and morphogenesis. Indeed, the phosphorylated pathway is strictly linked to cancer. Moreover, L-serine has been suggested as a ready-to-use treatment, as also recently proposed for Alzheimer's disease. Here, we present our current state of knowledge concerning the three mammalian enzymes of the phosphorylated pathway and known mutations related to pathological conditions: although the structure of these enzymes has been solved, how enzyme activity is regulated remains largely unknown. We believe that an in-depth investigation of these enzymes is crucial to identify the molecular mechanisms involved in modulating concentrations of the serine enantiomers and for studying the interplay between glial and neuronal cells and also to determine the most suitable therapeutic approach for various diseases.

Postsynaptic Serine Racemase Regulates NMDA Receptor Function

d-serine is the primary NMDAR coagonist at mature forebrain synapses and is synthesized by the enzyme serine racemase (SR). However, our understanding of the mechanisms regulating the availability of synaptic d-serine remains limited. Though early studies suggested d-serine is synthesized and released from astrocytes, more recent studies have demonstrated a predominantly neuronal localization of SR. More specifically, recent work intriguingly suggests that SR may be found at the postsynaptic density, yet the functional implications of postsynaptic SR on synaptic transmission are not yet known. Here, we show an age-dependent dendritic and postsynaptic localization of SR and d-serine by immunohistochemistry and electron microscopy in mouse CA1 pyramidal neurons. In addition, using a single-neuron genetic approach in SR conditional KO mice from both sexes, we demonstrate a cell-autonomous role for SR in regulating synaptic NMDAR function at Schaffer collateral (CA3)-CA1 synapses. Importantly, single-neuron genetic deletion of SR resulted in the elimination of LTP at 1 month of age, which could be rescued by exogenous d-serine. Interestingly, there was a restoration of LTP by 2 months of age that was associated with an upregulation of synaptic GluN2B. Our findings support a cell-autonomous role for postsynaptic neuronal SR in regulating synaptic NMDAR function and suggests a possible autocrine mode of d-serine action.SIGNIFICANCE STATEMENT NMDARs are key regulators of neurodevelopment and synaptic plasticity and are unique in their requirement for binding of a coagonist, which is d-serine at most forebrain synapses. However, our understanding of the mechanisms regulating synaptic d-serine availability remains limited. d-serine is synthesized in the brain by the neuronal enzyme serine racemase (SR). Here, we show dendritic and postsynaptic localization of SR and d-serine in CA1 pyramidal neurons. In addition, using single-neuron genetic deletion of SR, we establish a role of postsynaptic SR in regulating NMDAR function. These results support an autocrine mode of d-serine action at synapses.

Physiology of Astroglial Excitability

Classic physiology divides all neural cells into excitable neurons and nonexcitable neuroglia. Neuroglial cells, chiefly responsible for homeostasis and defense of the nervous tissue, coordinate their complex homeostatic responses with neuronal activity. This coordination reflects a specific form of glial excitability mediated by complex changes in intracellular concentration of ions and second messengers organized in both space and time. Astrocytes are equipped with multiple molecular cascades, which are central for regulating homeostasis of neurotransmitters, ionostasis, synaptic connectivity, and metabolic support of the central nervous system. Astrocytes are further provisioned with multiple receptors for neurotransmitters and neurohormones, which upon activation trigger intracellular signals mediated by Ca²⁺, Na⁺, and cyclic AMP. Calcium signals have distinct organization and underlying mechanisms in different astrocytic compartments thus allowing complex spatiotemporal signaling. Signals mediated by fluctuations in cytosolic Na⁺ are instrumental for coordination of Na⁺ dependent astrocytic transporters with tissue state and homeostatic demands. Astroglial ionic excitability may also involve K⁺, H^+, and Cl^-. The cyclic AMP signalling system is, in comparison to ions, much slower in targeting astroglial effector mechanisms. This evidence review summarizes the concept of astroglial intracellular excitability.

l-Serine links metabolism with neurotransmission

Brain energy metabolism is often considered as a succession of biochemical steps that metabolize the fuel (glucose and oxygen) for the unique purpose of providing sufficient ATP to maintain the huge information processing power of the brain. However, a significant fraction (10-15 %) of glucose is shunted away from the ATP-producing pathway (oxidative phosphorylation) and may be used to support other functions. Recent studies have pointed to the marked compartmentation of energy metabolic pathways between neurons and glial cells. Here, we focused our attention on the biosynthesis of l-serine, a non-essential amino acid that is formed exclusively in glial cells (mostly astrocytes) by re-routing the metabolic fate of the glycolytic intermediate, 3-phosphoglycerate (3PG). This metabolic pathway is called the phosphorylated pathway and transforms 3PG into l-serine via three enzymatic reactions. We first compiled the available data on the mechanisms that regulate the flux through this metabolic pathway. We then reviewed the current evidence that is beginning to unravel the roles of l-serine both in the healthy and diseased brain, leading to the notion that this specific metabolic pathway connects glial metabolism with synaptic activity and plasticity. We finally suggest that restoring astrocyte-mediated l-serine homeostasis may provide new therapeutic strategies for brain disorders.

PHGDH supports liver ceramide synthesis and sustains lipid homeostasis

BACKGROUND

d-3-phosphoglycerate dehydrogenase (PHGDH), which encodes the first enzyme in serine biosynthesis, is overexpressed in human cancers and has been proposed as a drug target. However, whether PHGDH is critical for the proliferation or homeostasis of tissues following the postnatal period is unknown.

METHODS

To study PHGDH inhibition in adult animals, we developed a knock-in mouse model harboring a PHGDH shRNA under the control of a doxycycline-inducible promoter. With this model, PHGDH depletion can be globally induced in adult animals, while sparing the brain due to poor doxycycline delivery.

RESULTS

We found that PHGDH depletion is well tolerated, and no overt phenotypes were observed in multiple highly proliferative cell compartments. Further, despite detectable knockdown and impaired serine synthesis, liver and pancreatic functions were normal. Interestingly, diminished PHGDH expression reduced liver serine and ceramide levels without increasing the levels of deoxysphingolipids. Further, liver triacylglycerol profiles were altered, with an accumulation of longer chain, polyunsaturated tails upon PHGDH knockdown.

CONCLUSIONS

These results suggest that dietary serine is adequate to support the function of healthy, adult murine tissues, but PHGDH-derived serine supports liver ceramide synthesis and sustains general lipid homeostasis.

D-Serine as the gatekeeper of NMDA receptor activity: implications for the pharmacologic management of anxiety disorders

Fear, anxiety, and trauma-related disorders, including post-traumatic stress disorder (PTSD), are quite common and debilitating, with an estimated lifetime prevalence of ~28% in Western populations. They are associated with excessive fear reactions, often including an inability to extinguish learned fear, increased avoidance behavior, as well as altered cognition and mood. There is an extensive literature demonstrating the importance of N-methyl-D-aspartate receptor (NMDAR) function in regulating these behaviors. NMDARs require the binding of a co-agonist, D-serine or glycine, at the glycine modulatory site (GMS) to function. D-serine is now garnering attention as the primary NMDAR co-agonist in limbic brain regions implicated in neuropsychiatric disorders. L-serine is synthesized by astrocytes, which is then transported to neurons for conversion to D-serine by serine racemase (SR), a model we term the 'serine shuttle.' The neuronally-released D-serine is what regulates NMDAR activity. Our review discusses how the systems that regulate the synaptic availability of D-serine, a critical gatekeeper of NMDAR-dependent activation, could be targeted to improve the pharmacologic management of anxiety-related disorders where the desired outcomes are the facilitation of fear extinction, as well as mood and cognitive enhancement.

D-Serine, the Shape-Shifting NMDA Receptor Co-agonist

Shape-shifting, a phenomenon wide-spread in folklore, refers to the ability to physically change from one identity to another, typically from an innocuous entity to a destructive one. The amino acid D-serine over the last 25 years has "shape-shifted" into several identities: a purported glial transmitter activating N-methyl-D-aspartate receptors (NMDARs), a co-transmitter concentrated in excitatory glutamatergic neurons, an autocrine that is released at dendritic spines to prime their post-synaptic NMDARs for an instantaneous response to glutamate and an excitotoxic moiety released from inflammatory (A1) astrocytes. This article will review evidence in support of these scenarios and the artifacts that misled investigators of the true identity of D-serine.

Serine-dependent redox homeostasis regulates glioblastoma cell survival

Background

The amino acid serine is an important substrate for biosynthesis and redox homeostasis. We investigated whether glioblastoma (GBM) cells are dependent on serine for survival under conditions of the tumour microenvironment.

Methods

Serine availability in GBM cells was modulated pharmacologically, genetically and by adjusting serine and glycine concentrations in the culture medium. Cells were investigated for regulation of serine metabolism, proliferation, sensitivity to hypoxia-induced cell death and redox homeostasis.

Results

Hypoxia-induced expression of phosphoglycerate dehydrogenase (PHGDH) and the mitochondrial serine hydroxymethyltransferase (SHMT2) was observed in three of five tested glioma cell lines. Nuclear factor erythroid 2-related factor (Nrf) 2 activation also induced PHGDH and SHMT2 expression in GBM cells. Low levels of endogenous PHGDH as well as PHGDH gene suppression resulted in serine dependency for cell growth. Pharmacological inhibition of PHGDH with CBR-5884 reduced proliferation and sensitised cells profoundly to hypoxia-induced cell death. This effect was accompanied by an increase in reactive oxygen species and a decrease in the NADPH/NADP⁺ ratio. Similarly, hypoxia-induced cell death was enhanced by PHGDH gene suppression and reduced by PHGDH overexpression.

Conclusions

Serine facilitates adaptation of GBM cells to conditions of the tumour microenvironment and its metabolism could be a plausible therapeutic target.

Impairment of Glycolysis-Derived l-Serine Production in Astrocytes Contributes to Cognitive Deficits in Alzheimer's Disease

Alteration of brain aerobic glycolysis is often observed early in the course of Alzheimer's disease (AD). Whether and how such metabolic dysregulation contributes to both synaptic plasticity and behavioral deficits in AD is not known. Here, we show that the astrocytic l-serine biosynthesis pathway, which branches from glycolysis, is impaired in young AD mice and in AD patients. l-serine is the precursor of d-serine, a co-agonist of synaptic NMDA receptors (NMDARs) required for synaptic plasticity. Accordingly, AD mice display a lower occupancy of the NMDAR co-agonist site as well as synaptic and behavioral deficits. Similar deficits are observed following inactivation of the l-serine synthetic pathway in hippocampal astrocytes, supporting the key role of astrocytic l-serine. Supplementation with l-serine in the diet prevents both synaptic and behavioral deficits in AD mice. Our findings reveal that astrocytic glycolysis controls cognitive functions and suggest oral l-serine as a ready-to-use therapy for AD.

Silk sericin intake leads to increases in l-serine and l-tyrosine levels in the mouse brain and the simultaneous facilitation of brain noradrenergic turnover

Sericin is a protein component of the silkworm cocoon, and contains a high proportion of L-serine, but it has been mostly disposed of as an industrial waste. However, recent studies have revealed its unique biological functionalities beneficial to human health. This study aimed to evaluate the effect of acute oral intake of sericin on amino acid and neurotransmitter metabolism in the mouse brain. Acute administration of chemically modified sericin (0.26 g/30 g body weight) increased L-serine and L-tyrosine levels in the serum and brain, although the L-tyrosine content in the sericin was less than 3% (w/w). In addition, sericin administration led to a significant facilitation of noradrenergic turnover via enhancement of 3-methoxy-4-hydroxyphenylethyleneglycol, a principal metabolite of noradrenaline, in several of the brain regions examined. These present findings suggest that oral intake of sericin efficiently delivers L-serine and L-tyrosine to the brain, thus stimulating noradrenergic activity in the brain.

Abbreviations: DA: dopamine; 5-HIAA: 5-hydroxyindoleicetic acid; 5-HT: 5-hydroxytryptamine; HVA: homovanillic acid; MHPG: 3-methoxy-4-hydroxyphenylethyleneglycol; 3-MT: 3-methoxytyramine; NA: noradrenaline; NM: normetanephrine; Veh: vehicle

Na+-dependent transporters: The backbone of astroglial homeostatic function

Astrocytes are the principal homeostatic cells of the central nerves system (CNS) that support the CNS function at all levels of organisation, from molecular to organ. Several fundamental homeostatic functions of astrocytes are mediated through plasmalemmal pumps and transporters; most of which are also regulated by the transplasmalemmal gradient of Na⁺ ions. Neuronal activity as well as mechanical or chemical stimulation of astrocytes trigger plasmalemmal Na⁺ fluxes, which in turn generate spatio-temporally organised transient changes in the cytosolic Na⁺ concentration, which represent the substrate of astroglial Na⁺ signalling. Astroglial Na⁺ signals link and coordinate neuronal activity and CNS homeostatic demands with the astroglial homeostatic response.

Genome-wide CRISPR/Cas9 library screening identified PHGDH as a critical driver for Sorafenib resistance in HCC

Sorafenib is the standard treatment for advanced hepatocellular carcinoma (HCC). However, the development of drug resistance is common. By using genome-wide CRISPR/Cas9 library screening, we identify phosphoglycerate dehydrogenase (PHGDH), the first committed enzyme in the serine synthesis pathway (SSP), as a critical driver for Sorafenib resistance. Sorafenib treatment activates SSP by inducing PHGDH expression. With RNAi knockdown and CRISPR/Cas9 knockout models, we show that inactivation of PHGDH paralyzes the SSP and reduce the production of αKG, serine, and NADPH. Concomitantly, inactivation of PHGDH elevates ROS level and induces HCC apoptosis upon Sorafenib treatment. More strikingly, treatment of PHGDH inhibitor NCT-503 works synergistically with Sorafenib to abolish HCC growth in vivo. Similar findings are also obtained in other FDA-approved tyrosine kinase inhibitors (TKIs), including Regorafenib or Lenvatinib. In summary, our results demonstrate that targeting PHGDH is an effective approach to overcome TKI drug resistance in HCC.

Resistance to the tyrosine kinase inhibitor Sorafenib, which is the standard treatment for advanced hepatocellular carcinoma, is a major clinical challenge. Here, the authors show that phosphoglycerate dehydrogenase, a key enzyme in the serine synthesis pathway, drives sorafenib resistance.

Neurotoxic astrocytes express the d-serine synthesizing enzyme, serine racemase, in Alzheimer's disease

Although β-amyloid plaques are a well-recognized hallmark of Alzheimer's disease (AD) neuropathology, no drugs reducing amyloid burden have shown efficacy in clinical trials, suggesting that once AD symptoms emerge, disease progression becomes independent of Aβ production. Reactive astrocytes are another neuropathological feature of AD, where there is an emergence of neurotoxic (A1) reactive astrocytes. We find that serine racemase (SR), the neuronal enzyme that produces the N-methyl-d-aspartate receptor (NMDAR) co-agonist d-serine, is robustly expressed in A1-reactive neurotoxic astrocytes in the hippocampus and entorhinal cortex of AD subjects and an AD rat model. Furthermore, we observe intracellular signaling changes consistent with increased extra-synaptic NMDAR activation, excitotoxicity and decreased neuronal survival. Thus, reducing neurotoxic d-serine release from A1 inflammatory astrocytes could have therapeutic benefit for mild to advanced AD, when anti-amyloid strategies are ineffective.

The NMDA receptor activation by d-serine and glycine is controlled by an astrocytic Phgdh-dependent serine shuttle

Astrocytes express the 3-phosphoglycerate dehydrogenase (Phgdh) enzyme required for the synthesis of l-serine from glucose. Astrocytic l-serine was proposed to regulate NMDAR activity by shuttling to neurons to sustain d-serine production, but this hypothesis remains untested. We now report that inhibition of astrocytic Phgdh suppressed the de novo synthesis of l-and d-serine and reduced the NMDAR synaptic potentials and long-term potentiation (LTP) at the Schaffer collaterals-CA1 synapse. Likewise, enzymatic removal of extracellular l-serine impaired LTP, supporting an l-serine shuttle mechanism between glia and neurons in generating the NMDAR coagonist d-serine. Moreover, deletion of serine racemase (SR) in glutamatergic neurons abrogated d-serine synthesis to the same extent as Phgdh inhibition, suggesting that neurons are the predominant source of the newly synthesized d-serine. We also found that the synaptic NMDAR activation in adult SR-knockout (KO) mice requires Phgdh-derived glycine, despite the sharp decline in the postnatal glycine levels as a result of the emergence of the glycine cleavage system. Unexpectedly, we also discovered that glycine regulates d-serine metabolism by a dual mechanism. The first consists of tonic inhibition of SR by intracellular glycine observed in vitro, primary cultures, and in vivo microdialysis. The second involves a transient glycine-induce d-serine release through the Asc-1 transporter, an effect abolished in Asc-1 KO mice and diminished by deleting SR in glutamatergic neurons. Our observations suggest that glycine is a multifaceted regulator of d-serine metabolism and implicate both d-serine and glycine in mediating NMDAR synaptic activation at the mature hippocampus through a Phgdh-dependent shuttle mechanism.

l-Serine dietary supplementation is associated with clinical improvement of loss-of-function GRIN2B-related pediatric encephalopathy

Autosomal dominant mutations in GRIN2B are associated with severe encephalopathy, but little is known about the pathophysiological outcomes and any potential therapeutic interventions. Genetic studies have described the association between de novo mutations of genes encoding the subunits of the N-methyl-d-aspartate receptor (NMDAR) and severe neurological conditions. Here, we evaluated a missense mutation in GRIN2B, causing a proline-to-threonine switch (P553T) in the GluN2B subunit of NMDAR, which was found in a 5-year-old patient with Rett-like syndrome with severe encephalopathy. Structural molecular modeling predicted a reduced pore size of the mutant GluN2B-containing NMDARs. Electrophysiological recordings in a HEK-293T cell line expressing the mutated subunit confirmed this prediction and showed an associated reduced glutamate affinity. Moreover, GluN2B(P553T)-expressing primary murine hippocampal neurons showed decreased spine density, concomitant with reduced NMDA-evoked currents and impaired NMDAR-dependent insertion of the AMPA receptor subunit GluA1 at stimulated synapses. Furthermore, the naturally occurring coagonist d-serine restored function to GluN2B(P553T)-containing NMDARs. l-Serine dietary supplementation of the patient was hence initiated, resulting in the increased abundance of d-serine in the plasma and brain. The patient has shown notable improvements in motor and cognitive performance and communication after 11 and 17 months of l-serine dietary supplementation. Our data suggest that l-serine supplementation might ameliorate GRIN2B-related severe encephalopathy and other neurological conditions caused by glutamatergic signaling deficiency.

Glutamate hypothesis in schizophrenia

Schizophrenia is a chronic and severe psychiatric disorder that has profound impact on an individual's life and on society. Thus, developing more effective therapeutic interventions is essential. Over the past quarter-century, an abundance of evidence from pharmacologic challenges, post-mortem studies, brain imaging, and genetic studies supports the role of glutamatergic dysregulation in the pathophysiology of schizophrenia, and the results of recent randomized clinical trials based on this evidence have yielded promising results. In this article, we review the evidence that alterations in glutamatergic neurotransmission, especially focusing on the N-methyl-d-aspartate receptor (NMDAR) function, may be a critical causative feature of schizophrenia, how this contributes to pathologic circuit function in the brain, and how these insights are revealing whole new avenues for treatment development that could reduce treatment-resistant symptoms, which account for persistent disability.

Protein interactome of the deamidase phosphoribosylformylglycinamidine synthetase (PFAS) by LC-MS/MS

Phosphoribosylformylglycinamidine synthase (PFAS) is an essential enzyme in de novo synthesis of purine. Previously, PFAS has been reported to modulate RIG-I activation during viral infection via deamidation. In this study, we sought to identify potential substrates that PFAS can deamidate. Flag-PFAS was transfected into HEK-293T cells and PFAS associated proteins were purified with anti-Flag M2 magnetic beads. PFAS associated proteins were identified using mass spectrometry and were analyzed using bioinformatics tools including KEGG pathway analysis, gene ontology annotation, and protein interaction network analysis. A total of 441 proteins is suggested to potentially interact with PFAS. Of this number, 12 were previously identified and 429 are newly identified. The interactions of PFAS with CAD, CCT2, PRDX1, and PHGDH were confirmed by co-immunoprecipitation and western blotting. This study is first to report the interaction of PFAS with several proteins which play physiological roles in tumor development including CAD, CCT2, PRDX1, and PHGDH. Furthermore, we show here that PFAS is able to deamidate PHGDH, and induce other posttranslational modification into CAD, CCT2 and PRDX1. The present data provide insight on the biological function of PFAS. Further study to explore the role of these protein interactions in tumorigenesis and other diseases is recommended.

D-3-Phosphoglycerate Dehydrogenase

l-Serine is the immediate precursor of d-serine, a major agonist of the N-methyl-d-aspartate (NMDA) receptor. l-Serine is a pivotal amino acid since it serves as a precursor to a large number of essential metabolites besides d-serine. In all non-photosynthetic organisms, including mammals, a major source of l-serine is the phosphorylated pathway of l-serine biosynthesis. The pathway consists of three enzymes, d-3-phosphoglycerate dehydrogenase (PGDH), phosphoserine amino transferase (PSAT), and l-phosphoserine phosphatase (PSP). PGDH catalyzes the first step in the pathway by converting d-3-phosphoglycerate (PGA), an intermediate in glycolysis, to phosphohydroxypyruvate (PHP) concomitant with the reduction of NAD⁺. In some, but not all organisms, the catalytic activity of PGDH can be regulated by feedback inhibition by l-serine. Three types of PGDH can be distinguished based on their domain structure. Type III PGDHs contain only a nucleotide binding and substrate binding domain. Type II PGDHs contain an additional regulatory domain (ACT domain), and Type I PGDHs contain a fourth domain, termed the ASB domain. There is no consistent pattern of domain content that correlates with organism type, and even when additional domains are present, they are not always functional. PGDH deficiency results in metabolic defects of the nervous system whose systems range from microcephaly at birth, seizures, and psychomotor retardation. Although deficiency of any of the pathway enzymes have similar outcomes, PGDH deficiency is predominant. Dietary or intravenous supplementation with l-serine is effective in controlling seizures but has little effect on psychomotor development. An increase in PGDH levels, due to overexpression, is also associated with a wide array of cancers. In culture, PGDH is required for tumor cell proliferation, but extracellular l-serine is not able to support cell proliferation. This has led to the hypothesis that the pathway is performing some function related to tumor growth other than supplying l-serine. The most well-studied PGDHs are bacterial, primarily from Escherichia coli and Mycobacterium tuberculosis, perhaps because they have been of most interest mechanistically. However, the relatively recent association of PGDH with neuronal defects and human cancers has provoked renewed interest in human PGDH.

Impact of Aging in Microglia-Mediated D-Serine Balance in the CNS

A mild chronic inflammatory state, like that observed in aged individuals, affects microglial function, inducing a dysfunctional phenotype that potentiates neuroinflammation and cytotoxicity instead of neuroprotection in response to additional challenges. Given that inflammatory activation of microglia promotes increased release of D-serine, we postulate that age-dependent inflammatory brain environment leads to microglia-mediated changes on the D-serine-regulated glutamatergic transmission. Furthermore, D-serine dysregulation, in addition to affecting synaptogenesis and synaptic plasticity, appears also to potentiate NMDAR-dependent excitotoxicity, promoting neurodegeneration and cognitive impairment. D-serine dysregulation promoted by microglia could have a role in age-related cognitive impairment and in the induction and progression of neurodegenerative processes like Alzheimer's disease.