Cofactors of Serine Racemase That Physiologically Stimulate the Synthesis of the N-Methyl-D-Aspartate (NMDA) Receptor Coagonist D-Serine
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Cofactors of serine racemase that physiologically stimulate the synthesis of the N-methyl-D-aspartate (NMDA) receptor coagonist D-serine Joari de Miranda*†, Rogerio Panizzutti*†, Veronika N. Foltyn‡, and Herman Wolosker‡§ *Department of Biochemistry, Federal University of Rio de Janeiro, Rio de Janeiro RJ 21941-590, Brazil; and ‡Department of Biochemistry, Technion-Israel Institute of Technology, The Bruce Rappaport Faculty of Medicine, Haifa 31096, Israel Edited by Solomon H. Snyder, Johns Hopkins University School of Medicine, Baltimore, MD, and approved September 9, 2002 (received for review July 16, 2002) High levels of D-serine occur in the brain, challenging the notion identification of cellular cofactors of the enzyme. We found that ϩ that D-amino acids would not be present or play a role in mammals. Mg2 and ATP are physiological ligands of SR that increase D-serine levels in the brain are even higher than many L-amino severalfold the production of D-serine and elicit considerable acids, such as asparagine, valine, isoleucine, and tryptophan, pyruvate generation by direct deamination of L-serine. Our among others. D-serine is synthesized by a serine racemase (SR) present data reveal a link between D-amino acid synthesis and a enzyme, which directly converts L-toD-serine. We now report that new metabolic route for production of pyruvate in the brain. SR is a bifunctional enzyme, producing both D-serine and pyruvate in cultured cells and in vitro. Transfection of SR into HEK 293 cells Materials and Methods elicits synthesis of D-serine and augmented release of pyruvate to Materials. Adenine nucleotides, L- and D-amino acids, aminooxy- culture media. We identified substances present in HEK 293 and acetic acid, apyrase (grade VII), catalase, glutathione-agarose, astrocyte cell extracts that strongly stimulate D-serine production NADH, pyridoxal 5Ј-phosphate (PLP), and Tris were obtained by SR and elicit production of pyruvate. Experiments with recom- from Sigma. DMEM, FBS, glutamine, penicillin–streptomycin, ؉ binant enzyme reveal that Mg2 and ATP present in the cell and trypsin solutions were obtained from Biological Industries extracts are physiological cofactors and increase 5- to 10-fold the (Kibbutz Beit Haemek, Israel). D-amino acid oxidase from pig rates of racemization and production of pyruvate. As much as three kidney (EC 1.4.3.3) and lactate dehydrogenase were obtained molecules of pyruvate are synthesized for each molecule of from Roche Molecular Biochemicals. L-[14C(U)]serine was pur- D-serine produced by SR. This finding constitutes a previously chased from Perkin–Elmer. Thrombin cleavage capture kit was undescribed mechanism underlying D-amino acid synthesis in purchased from Novagen. Other reagents were of analytical mammals, different from classical amino acid racemases present in grade. bacteria. Our data link the production of D-serine to the energy metabolism, with implications for the metabolic and transmitter Stable Expression in HEK 293 Cells. Mouse full-length SR was crosstalk between glia and neurons. subcloned into pCMV-GST expression vector containing a cytomegalovirus promoter and the gene for the GST of Schis- glutamate receptors ͉ pyruvate ͉ astrocytes tosoma japonicum (6). The construct was cotransfected into HEK 293 cells with a plasmid containing the gene for resistance n recent years, it has been shown that astrocytes and neurons to neomycin (PIRESneo, CLONTECH). Resistant clones were Iexhibit a dynamic bidirectional signaling that profoundly in- selected by using 400 g͞ml G418 (Life Technologies, Grand fluences neuronal activity and development (1). Astrocytes Island, NY), and several independent colonies were isolated and modulate synaptic transmission by releasing chemical transmit- expanded. Clones expressing highest levels of SR were selected ters and eliciting Ca2ϩ waves in nearby neurons. The search for by Western blot analysis using a polyclonal antibody against SR. new transmitter molecules in the brain uncovered the presence The cells were maintained in DMEM plus 10% FBS supple- of high levels of D-serine in astrocytes, a D-amino acid not mented with 200 g͞ml G418. previously thought to occur in mammals (2, 3). D-serine is synthe- sized by a glial-enriched enzyme serine racemase (SR), which Purification of Recombinant SR. Stable HEK 293 cell line express- directly converts L-to D-serine (4–7). SR does not bear significant ing SR-GST was cultured in DMEM plus 10% FBS. Cells grown homology with bacterial racemases, and little is known about the in 140-mm dishes were harvested and lysed in medium contain- mechanism and regulation of D-serine production. ing 20 mM Tris⅐HCl (pH 7.4), 500 mM NaCl, 1% Triton X-100, D-serine released from astrocytes seems to be an endogenous 0.2 mM PMSF, 2 mM EDTA, 2 mM DTT, and 15 M PLP. The ligand of the N-methyl-D-aspartate (NMDA) receptor (3, 8). lysate was centrifuged at 40,000 ϫ g for 20 min at 4°C to remove Depletion of endogenous D-serine in slices and cultured cells insoluble material. The supernatant was incubated overnight strongly diminishes NMDA receptor responses measured bio- under slow rotation at 4°C with glutathione-agarose beads to chemically and electrophysiologically (8). Massive stimulation of bind SR-GST fusion protein. The beads were washed six times NMDA receptors is implicated in neural damage after stroke (9), with PBS supplemented with 0.5% Triton X-100͞300 mM and inhibitors of SR may be useful to prevent stroke damage. NaCl͞15 M PLP. The beads were subsequently washed four Thus, inhibitors of SR provide a strategy to decrease NMDA times with PBS plus 15 M PLP to remove the salt and detergent. receptor coactivation and may be useful in conditions in which SR-GST fusion protein was eluted by batch incubation with 40 overstimulation of NMDA receptors plays a pathological role. To clarify the role of D-serine as a modulator of NMDA receptors, one should identify the factors that regulate SR and This paper was submitted directly (Track II) to the PNAS office. D-serine signaling. In this paper, we explored mechanisms Abbreviations: NMDA, N-methyl-D-aspartate; PLP, pyridoxal 5Ј-phosphate; SR, serine race- regulating the production of D-serine by SR. We demonstrate mase; TCA, trichloroacetic acid. that SR is a unique bifunctional enzyme, producing D-serine and †J.d.M and R.P. contributed equally to this work. pyruvate at the same time. This property was revealed by our §To whom correspondence should be addressed. E-mail: [email protected]. 14542–14547 ͉ PNAS ͉ October 29, 2002 ͉ vol. 99 ͉ no. 22 www.pnas.org͞cgi͞doi͞10.1073͞pnas.222421299 Downloaded by guest on September 28, 2021 mM Tris⅐HCl (pH 8.5), 10–20 mM reduced glutathione, 15 M Production of Deproteinized Cell Extracts. Confluent HEK 293 cells PLP, and 500 mM NaCl. The eluate was dialyzed for 2 h against cultured in 6-well plates were washed twice with cold PBS. The PBS plus 15 M PLP. This preparation was used for most cell content of one well was scraped off the plate and transferred experiments. Typically, 0.8–1.4 mg of SR-GST was obtained to a 1.5-ml tube on ice. The suspension was centrifuged for 15 s from 40 confluent HEK 293 culture dishes of 140-mm diameter. at 20,000 ϫ g, and the supernatant was discarded. The cell pellet In some experiments, the GST part was removed by incubating was suspended in 0.1 ml of cold PBS, and the protein was beads containing SR-GST with biotinylated thrombin for 16 h at removed by centrifugation after precipitation with 5% TCA. The room temperature in PBS plus 15 M PLP. Purified SR was acid was further extracted with diethyl ether, and the aqueous obtained after GST and biotinylated thrombin were removed phase containing soluble nonproteinaceous molecules was neu- with glutathione-agarose and streptavidin-agarose beads, re- tralized with NaOH. The samples were centrifuged at vacuum for spectively. The very high purity of this preparation has been 5 min to remove any residual diethyl ether. For deproteinized described elsewhere (6). extracts from astrocytes, we prepared primary astrocyte cultures in 6-well plates as described (13). After 14 days in culture, the Cell Transfection. HEK 293 cells were split into 6-well tissue cells were scraped off the plate, and the protein was removed by culture plates (Nunc) at 70–90% confluence. On the next day, TCA precipitation as described above for HEK 293 extracts. cells were transfected with 0.4 g of mouse full-length SR gene, a catalytically inactive mutant constructed by replacing Lys-56 Activity Assays. Reaction media contained 20 mM Tris⅐HCl (pH with glycine (5) or GFP cloned into pRK5-KS by using Effectene 7.4), 15 M PLP, and 10 mM L-serine in a final reaction volume reagent (Qiagen, Valencia, CA). Cells were used for experiments of 60 l. The reaction was started by addition of recombinant SR 2 days after transfection. Cells transfected with GFP revealed ͞ Ϸ (0.15 mg ml in PBS) and stopped after 1–2hat37°C by boiling 30% transfection efficiency monitored by fluorescence acti- for 5 min. Blanks were carried out with heat-inactivated enzyme. vated cell sorting (FACSCalibur, Becton Dickinson). To test the effects of deproteinized cell extracts on SR activity, the reaction was carried out in the presence of 30 l of extracts, Assay of D- and L-Serine in Cells. HEK 293-transfected cells were keeping the final reaction volume at 60 l. D-serine was moni- incubated in 6-well plates with 0.8 ml of DMEM, which contains tored by HPLC analysis. Formation of pyruvate from L-serine about 0.6 mM L-serine and 6 M contaminating D-serine.