A new strategy to decrease N-methyl-D-aspartate (NMDA) receptor coactivation: Inhibition of D-serine synthesis by converting serine racemase into an eliminase Roge´ rio Panizzutti*, Joari De Miranda*, Ca´ tia S. Ribeiro*, Simone Engelender†, and Herman Wolosker*‡ *Departamento de Bioquimica Medica, Instituto de Ciencias Biomedicas, and †Center for Neurodegenerative Diseases, Departamento de Anatomia, Universidade Federal do Rio de Janeiro, Rio de Janeiro, RJ 21491-590, Brazil Edited by Solomon H. Snyder, Johns Hopkins University School of Medicine, Baltimore, MD, and approved February 23, 2001 (received for review January 3, 2001) Serine racemase is a brain-enriched enzyme that synthesizes D- serine racemase genes (9, 10). The distribution of serine race- serine, an endogenous modulator of the glycine site of N-methyl- mase was closely similar to that of endogenous D-serine with the D-aspartate (NMDA) receptors. We now report that serine race- highest concentrations in the forebrain and negligible levels in mase catalyzes an elimination reaction toward a nonphysiological the brainstem. Both D-serine and serine racemase occur in substrate that provides a powerful tool to study its neurobiological astrocytes, in regions enriched in NMDA receptors, suggesting role and will be useful to develop selective enzyme inhibitors. that serine racemase physiologically synthesizes D-serine to Serine racemase catalyzes robust elimination of L-serine O-sulfate regulate NMDA receptor activity (9). that is 500 times faster than the physiological racemization reac- As an endogenous coagonist of NMDA receptors, D-serine tion, generating sulfate, ammonia, and pyruvate. This reaction may play a role in several pathological conditions related to provides the most simple and sensitive assay to detect the enzyme NMDA receptor dysfunction. Elevations of extracellular con- activity so far. We establish stable cell lines expressing serine centrations of D-serine are observed after transient cerebral racemase and show that serine racemase can also be converted ischemia in rats (11), and drugs that block the ‘‘glycine site’’ of into a powerful eliminase in cultured cells, while the racemization NMDA receptors prevent stroke damage (12, 13). D-serine is at of L-serine is inhibited. Likewise, L-serine O-sulfate inhibits the least as potent as glycine in stimulating glutamate-induced synthesis of D-serine in primary astrocyte cultures. We conclude activation of NMDA receptors (3, 14), and massive stimulation that the synthetic compound L-serine O-sulfate is a better substrate of NMDA receptors is implicated in neural damage following than L-serine as well as an inhibitor of D-serine synthesis. Inhibition stroke (15). Thus, inhibitors of serine racemase may be useful in of serine racemase provides a new strategy to selectively decrease conditions such as stroke and neurodegenerative diseases where NMDA receptor coactivation and may be useful in conditions in overstimulation of NMDA receptors plays a pathological role. which overstimulation of NMDA receptors plays a pathological In addition to being a therapeutic target, the study of serine role. racemase biochemical properties and regulation are important to elucidate the neurobiological role of D-serine. However, this task has been challenging because of the difficulties involving the glutamate receptors ͉ L-serine O-sulfate routine detection of small amounts of D-serine by the methods available and by the lack of an active preparation of purified igh levels of D-serine, a D-amino acid not previously thought recombinant enzyme. In this report we discovered a new reaction Hto occur in mammals, have recently been found in mam- catalyzed by recombinant serine racemase that has been over- malian brain (1, 2). Several lines of evidence indicate that expressed and purified from mammalian cells. We found that D-serine is an endogenous modulator of the N-methyl-D- mouse serine racemase robustly destroys L-serine O-sulfate aspartate (NMDA) type of glutamate receptors. NMDA recep- through an elimination reaction. Degradation of L-serine O- tors require coactivation at a ‘‘glycine site’’ at which D-serine is sulfate is 500 times faster than the racemization of L-serine, at least as potent as glycine (3). Extracellular levels of endoge- demonstrating that serine racemase can function as a powerful nous D-serine are comparable to glycine in prefrontal cortex, eliminase. Additionally, detection of L-serine O-sulfate degra- whereas in the striatum, D-serine values are more than twice the dation products provides the most reliable and easy method to values for glycine as measured by in vivo microdialysis (4). measure the activity of serine racemase so far. We also show that Immunohistochemical studies show a high degree of colocaliza- serine racemase can function as an eliminase in cultured cells, tion of D-serine and NMDA receptors in the forebrain, whereas while D-serine synthesis is inhibited. The present findings have densities for glycine are more prominent in the brainstem (5, 6). implications for the development of selective inhibitors acting on D-serine occurs in protoplasmic astrocytes that ensheathe the serine racemase to decrease NMDA receptor coactivation. synapses and can be released from astrocyte cultures by activa- Materials and Methods tion of the kainate type of glutamate receptors (5). Direct evidence that D-serine normally mediates NMDA transmission Materials. Amino acids, aminooxyacetic acid (AOAA), L- comes from experiments showing that destruction of endoge- homocysteic acid, L-homoserine, luminol (sodium salt), phenyl- nous D-serine selectively by application of D-amino acid oxidase greatly reduces NMDA receptor activity monitored both bio- This paper was submitted directly (Track II) to the PNAS office. chemically and electrophysiologically in slices and cell culture preparations (7). Abbreviations: AAT, aspartate aminotransferase; AOAA, aminooxyacetic acid; GST, gluta- thione S-transferase; NMDA, N-methyl-D-aspartate; PLP, pyridoxal 5Ј-phosphate; Rac-WT, Because amino acid racemases were thought to be restricted wild-type serine racemase; Rac-K56G, catalytically inactive serine racemase mutant. to bacteria, the origin of D-serine has been puzzling. We recently ‡To whom reprint requests should be addressed. E-mail: [email protected]. showed that D-serine is synthesized from L-serine by a pyridoxal Ј The publication costs of this article were defrayed in part by page charge payment. This 5 -phosphate-dependent serine racemase. We purified serine article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. racemase from murine brain (8) and cloned mouse and human §1734 solely to indicate this fact. 5294–5299 ͉ PNAS ͉ April 24, 2001 ͉ vol. 98 ͉ no. 9 www.pnas.org͞cgi͞doi͞10.1073͞pnas.091002298 Downloaded by guest on September 27, 2021 by sonication in media containing 50 mM Tris⅐HCl (pH 7.4), 400 mM NaCl, 0.2 mM phenylmethylsulfonyl fluoride, 2 mM EDTA, 2mMDTT,and15␮M pyridoxal 5Ј-phosphate (PLP). Then, 0.1% Triton X-100 was added and the homogenate was centri- fuged at 40,000 ϫ g for 10 min at 4°C to remove membrane and other heavy fractions. The supernatant was incubated for 6 h with glutathione-Sepharose 4B beads to bind serine racemase– GST fusion protein. The beads were washed five times with PBS supplemented with 300 mM NaCl, 1 mM EDTA, 2 mM DTT, and 15 ␮M PLP. To obtain purified serine racemase, the GST part of the fusion protein was cleaved out by incubation with biotinylated thrombin for 16 h at room temperature in PBS containing 15 ␮M PLP. Biotinylated thrombin was removed with streptavidin-agarose according to the manufacturer’s instruc- tions (Novagen) and purified serine racemase was separated Fig. 1. Purification of active recombinant serine racemase from mamma- from the beads containing the GST part of the fusion protein by lian cells. Mouse serine racemase-GST fusion protein was expressed in centrifugation. Typically, 20–50 ␮g purified serine racemase are HEK293 cells and purified to homogeneity. (A) SDS͞PAGE (12%) stained with Coomassie brilliant blue. Lane 1, 40 ␮g transfected HEK293 cell obtained from each transfected 100-mm dish. protein extract; lane 2, 30 ␮g mouse serine racemase-GST fusion protein purified with glutathione-Sepharose beads; lane 3, 5 ␮g purified serine Serine Racemase Activity. Reaction media contained 50 mM racemase after GST portion was removed by cleavage with thrombin. (B) Tris⅐HCl (pH 8.2), 15 ␮M PLP, and 10 mM L-serine. The Serine racemase activity was measured at 37°C in media containing 50 mM reaction was started by addition of recombinant serine racemase Tris⅐HCl (pH 8.2), 10 ␮g͞ml recombinant serine racemase, 15 ␮M PLP, and (1–20 ␮g͞ml) and stopped by addition of 5% trichloroacetic acid 10 mM L-serine. Recombinant serine racemase exhibited robust racemase (TCA). The acid was subsequently extracted with diethyl ether activity (E), which was totally inhibited by 1 mM AOAA (F). The data are D representative of six different purifications. and -serine formed was monitored by a chemiluminescent method (8). L-serine used in the experiments was rendered free of contaminating D-serine as described (8). In experiments NEUROBIOLOGY methylsulfonyl fluoride, pyridoxal 5Ј-phosphate, pyruvate, L- testing the effects of several D-amino acids on serine racemase serine O-sulfate, and Tris were obtained from Sigma. D-amino activity, D-serine was monitored by HPLC analysis after deri- acid oxidase from pig kidney (EC 1.4.3.3), horseradish peroxi- vatization with o-phthaldialdehyde and N-tert-butyloxycarbonyl- dase and lactate dehydrogenase were obtained from Roche L-cysteine as described (17). Blanks were carried out with Molecular Biochemicals. Glutathione-Sepharose 4B was ob- heat-inactivated enzyme. tained from Amersham Pharmacia. A thrombin cleavage capture kit was purchased from Novagen. Other reagents were of Assay for L-Serine O-Sulfate Elimination. L-serine O-sulfate levels analytical grade. were monitored by the same HPLC technique we used for D-amino acid determination. L-serine O-sulfate was identified as Expression and Purification of Recombinant Serine Racemase.