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

Abstract

In mammalian brain, D-serine is synthesized from L-serine by serine racemase, and it functions as an obligatory coagonist at the glycine modulatory site of N-methyl-D-aspartate (NMDA)-selective glutamate receptors. Although diminution in D-serine level has been implicated in NMDA receptor hypofunction, which is thought to occur in schizophrenia, the source of the precursor L-serine and its role in D-serine metabolism in adult brain have yet to be determined. We investigated whether L-serine synthesized in brain via the phosphorylated pathway is essential for Dserine synthesis by generating mice with a conditional deletion of D-3-phosphoglycerate dehydrogenase (Phgdh; EC 1.1.1.95). This enzyme catalyzes the first step in L-serine synthesis via the phosphorylated pathway. HPLC analysis of serine enantiomers demonstrated that both L-and D-serine levels were markedly decreased in the cerebral cortex and hippocampus of conditional knock-out mice, whereas the serine deficiency did not alter protein expression levels of serine racemase and NMDA receptor subunits in these regions. The present study provides definitive proof that Lserine-synthesized endogenously via the phosphorylated pathway is a key rate-limiting factor for maintaining steadystate levels of D-serine in adult brain. Furthermore, NMDAevoked transcription of Arc, an immediate early gene, was diminished in the hippocampus of conditional knock-out mice. Thus, this study demonstrates that in mature neuronal circuits L-serine availability determines the rate of Dserine synthesis in the forebrain and controls NMDA receptor function at least in the hippocampus.Glutamate is the principal excitatory neurotransmitter in mammalian brain, acting on ionotropic and metabotropic glutamate receptors. Ionotropic glutamate receptors can be divided into three classes based on their preference for the ligands N-methyl-D-aspartate (NMDA), 3 ␣-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid, and kainate. Among these receptors, NMDA receptors have been implicated in synapse refinement, synaptic plasticity, and learning/memory as well as brain pathologies including excitotoxicity and psychiatric diseases (for review, see Refs. 1-3). Two NMDA receptor subunits, NR1 and NR2, form tetramers that comprise the functional receptors. For the NMDA receptor to function as a ligand-gated ion channel, a co-agonist must occupy the glycine modulatory site on NR1 coincident with glutamate binding to the transmitter recognition site on NR2 (4 -6).D-Serine occurs naturally in the adult brain of higher vertebrates and is particularly enriched in forebrain regions (7). This D-amino acid acts as an endogenous co-agonist at the glycine modulatory site of NMDA receptors (for review, see Refs. 8 -11). In the telencephalon, where D-serine is abundant (12), its distribution pattern resembles that of NR2A/B subunits (13). In contrast, immunoreactivity for free glycine is very low in telencephalon and is detected primarily in the hindbrain and hypothalamus, where NR2A/B expression is weaker than in ...

D-Serine in Glia and Neurons Derives from 3-Phosphoglycerate Dehydrogenase

“…Correspondingly, neurotoxicity associated with ischemia elicited by middle cerebral artery occlusion is reduced in Srr knock-out mice (Mustafa et al, 2010). The depletion of D-serine associated with Phgdh deletion is similar in magnitude to that after Srr deletion (Basu et al, 2009;Yang et al, 2010). Therefore, PHGDH inhibitors might offer benefit in attenuating neurotoxicity in vascular stroke and other diseases involving neurotoxicity linked to NMDAR overactivation.…”

“…Mice containing targeted mutations of Srr and radial glia/astrocyte-specific deletion of Phgdh have been described previously (Basu et al, 2009;Yang et al, 2010). BAC transgenic animals [Tg(Srr-EGFP)KJ355Gsat/Mmucd] were obtained from the GENSAT/Mutant Mouse Regional Resource Center.…”

Glycolytic flux controls d -serine synthesis through glyceraldehyde-3-phosphate dehydrogenase in astrocytes

“…The biosynthesis pathway of D-serine starts from a glycolytic intermediate, 3-phosphoglycerate, in the central nervous system. After three-step reactions from the intermediate, L-serine is synthesized in astrocytes and then converted into D-serine by SRR (30). Both serine enantiomers are deeply related to neurophysiology; L-serine is a neurotrophic factor essential for brain morphogenesis (31,32), whereas D-serine is a prerequisite for glutamatergic neurotransmission mediated by synaptic NMDA receptors (13,18,(33)(34)(35).…”

Down Regulation of Asparagine Synthetase and 3-Phosphoglycerate Dehydrogenase, and the Up-Regulation of Serine Dehydratase in Rat Liver from Intake of Excess Amount of Leucine Are Not Related to Leucine-Caused Amino Acid Imbalance

“…In the first step of the phosphorylation pathway, 3-phosphoglycerate derived from glycolysis is metabolized into phosphohydroxypyruvate by PHGDH, a rate-limiting enzyme. Yoshida et al have successfully generated PHGDH-knockout mice and demonstrated that systemic PHGDH-knockout embryos die 13.5 d post coitum (18), whereas brain-specific PHGDHknockout mice are viable despite microcephaly (19). These results clearly showed that the serine provided from the mother did not satisfy their requirement and the fetus needs the de novo synthesis of most of the serine for tissue growth and development.…”

“…Thus, sensitivity of the amino acids sensors may therefore have increased in the rats maintained on a 6% casein diet. It has been demonstrated that de novo synthesis of asparagine and serine was critical for cellular growth and function (16,(18)(19)(20)(21). ASNS and PHGDH are known to be expressed in several tissues at different levels.…”