Targeted disruption of serine racemase affects glutamatergic neurotransmission and behavior

Department of Psychiatry, Harvard Medical School, Belmont, MA 02478, USA.
Molecular Psychiatry (Impact Factor: 14.5). 09/2010; 15(11):1122-1122. DOI: 10.1038/mp.2010.84


A subset of glutamate receptors that are specifically sensitive to the glutamate analog N-methyl-D-aspartate (NMDA) are molecular coincidence detectors, necessary for activity-dependent processes of neurodevelopment and in sensory and cognitive functions. The activity of these receptors is modulated by the endogenous amino acid D-serine, but the extent to which D-serine is necessary for the normal development and function of the mammalian nervous system was previously unknown. Decreased signaling at NMDA receptors has been implicated in the pathophysiology of schizophrenia based on pharmacological evidence, and several human genes related to D-serine metabolism and glutamatergic neurotransmission have been implicated in the etiology of schizophrenia. Here we show that genetically modified mice lacking the ability to produce D-serine endogenously have profoundly altered glutamatergic neurotransmission, and relatively subtle but significant behavioral abnormalities that reflect hyperactivity and impaired spatial memory, and that are consistent with elevated anxiety.

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Available from: Richard Bergeron, Apr 14, 2014
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    • "Furthermore, SR−/− mice have many of the same brain morphological abnormalities observed in schizophrenia, such as reduced corticohippocampal volume, decreased dendritic spine density (Balu et al., 2012, 2013) and increased lateral ventricles (Puhl et al., 2015). Pharmacological NMDAR hypofunction models produce hyperlocomotion and stereotypy (Hoffman, 1992), as well as social withdrawal (Zou et al., 2008), behavioral abnormalities that are not observed in SR−/− mice (Basu et al., 2009; DeVito et al., 2011). Thus, the SR−/− mice exhibit a forebrain NMDA receptor hypofunction endophenotype with cortical neuronal atrophy associated with negative symptoms and cognitive but not with several behavioral abnormalities linked to psychosis. "
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    ABSTRACT: NMDA receptor (NMDAR) hypofunction is a compelling hypothesis for the pathophysiology of schizophrenia, because in part, NMDAR antagonists cause symptoms in healthy adult subjects that resemble schizophrenia. Therefore, NMDAR antagonists have been used as a method to induce NMDAR hypofunction in animals as a pharmacological model of schizophrenia. Serine racemase-null mutant (SR-/-) mice display constitutive NMDAR hypofunction due to the lack of d-serine. SR-/- mice have deficits in tropomyosin-related kinase receptor (TrkB)/Akt signaling and activity regulated cytoskeletal protein (Arc) expression, which mirror what is observed in schizophrenia. Thus, we analyzed these signaling pathways in MK801 sub-chronically (0.15mg/kg; 5days) treated adult wild-type mice. We found that in contrast to SR-/- mice, the activated states of downstream signaling molecules, but not TrkB, increased in MK801 treated mice. Furthermore, there is an age-dependent change in the behavioral reaction of people to NMDAR antagonists. We therefore administered the same dosing regimen of MK801 to juvenile mice and compared them to juvenile SR-/- mice. Our findings demonstrate that pharmacological NMDAR antagonism has different effects on TrkB/Akt signaling than genetically-induced NMDAR hypofunction. Given the phenotypic disparity between the MK801 model and schizophrenia, our results suggest that SR-/- mice more accurately reflect NMDAR hypofunction in schizophrenia. Copyright © 2014. Published by Elsevier B.V.
    Schizophrenia Research 01/2015; 162(1-3). DOI:10.1016/j.schres.2014.12.034 · 3.92 Impact Factor
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    • "D-Serine in the brain exists at high levels in areas where NMDAR, particularly the NR2B-subunit-containing receptors, are abundant [24] and regulates NMDAR as a potent and dominant coagonist over another coagonist, glycine [5], [59]. Because the mutant mice without the SR protein show a marked decrease in the expression level of D-serine (10–20% of WT mice) in the brain [33], [60], [61] and because D-serine distribution closely correlates with that of SR [2], [62], SR is a predominant enzyme for D-serine production [63]. Accordingly, SR has been considered an important enzyme for effective neurotransmission and synaptic plasticity via NMDAR activation. "
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    ABSTRACT: D-Serine, an endogenous coagonist of the N-methyl-D-aspartate receptor (NMDAR), is widely distributed in the central nervous system and is synthesized from L-serine by serine racemase (SR). NMDAR plays an important role in pain processing including central sensitization that eventually causes hyperalgesia. To elucidate the roles of D-serine and SR in pain transmission, we evaluated the behavioral changes and spinal nociceptive processing induced by formalin using SR knock-out (KO) mice. We found that SR is mainly distributed in lamina II of the dorsal horn of the spinal cord in wild-type (WT) mice. Although the formalin injected subcutaneously induced the biphasic pain response of licking in SR-KO and WT mice, the time spent on licking was significantly longer in the SR-KO mice during the second phase of the formalin test. The number of neurons immunopositive for c-Fos and phosphorylated extracellular signal-regulated kinase (p-ERK), which are molecular pain markers, in laminae I-II of the ipsilateral dorsal horn was significantly larger in the SR-KO mice. Immunohistochemical staining revealed that the distribution of SR changed from being broad to being concentrated in cell bodies after the formalin injection. On the other hand, the expression level of the cytosolic SR in the ipsilateral dorsal horn significantly decreased. Oral administration of 10 mM D-serine in drinking water for one week cancelled the difference in pain behaviors between WT and SR-KO mice in phase 2 of the formalin test. These findings demonstrate that the SR-KO mice showed increased sensitivity to inflammatory pain and the WT mice showed translocation of SR and decreased SR expression levels after the formalin injection, which suggest a novel antinociceptive mechanism via SR indicating an important role of D-serine in pain transmission.
    PLoS ONE 08/2014; 9(8):e105282. DOI:10.1371/journal.pone.0105282 · 3.23 Impact Factor
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    • "In fact, the D-serine level is inversely related to DAO activity in rodent brain (Morikawa et al., 2001; Wang and Zhu, 2003; Miyoshi et al., 2009). Since D-serine is an endogenous coagonist of N-methyl-D-aspartate (NMDA) glutamate receptors located at the synapse (Mothet et al., 2000; Panatier et al., 2006; Basu et al., 2009; Papouin et al., 2012), and also serves as an endogenous ligand at the δ2 glutamate receptor (Kakegawa et al., 2011), DAO has the potential to modulate neurotransmission through D-serine. "
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    ABSTRACT: It has been proposed that D-amino acid oxidase (DAO) plays an essential role in degrading D-serine, an endogenous coagonist of N-methyl-D-aspartate (NMDA) glutamate receptors. DAO shows genetic association with amyotrophic lateral sclerosis (ALS) and schizophrenia, in whose pathophysiology aberrant metabolism of D-serine is implicated. Although the pathology of both essentially involves the forebrain, in rodents, enzymatic activity of DAO is hindbrain-shifted and absent in the region. Here, we show activity-based distribution of DAO in the central nervous system (CNS) of humans compared with that of mice. DAO activity in humans was generally higher than that in mice. In the human forebrain, DAO activity was distributed in the subcortical white matter and the posterior limb of internal capsule, while it was almost undetectable in those areas in mice. In the lower brain centers, DAO activity was detected in the gray and white matters in a coordinated fashion in both humans and mice. In humans, DAO activity was prominent along the corticospinal tract, rubrospinal tract, nigrostriatal system, ponto-/olivo-cerebellar fibers, and in the anterolateral system. In contrast, in mice, the reticulospinal tract and ponto-/olivo-cerebellar fibers were the major pathways showing strong DAO activity. In the human corticospinal tract, activity-based staining of DAO did not merge with a motoneuronal marker, but colocalized mostly with excitatory amino acid transporter 2 and in part with GFAP, suggesting that DAO activity-positive cells are astrocytes seen mainly in the motor pathway. These findings establish the distribution of DAO activity in cerebral white matter and the motor system in humans, providing evidence to support the involvement of DAO in schizophrenia and ALS. Our results raise further questions about the regulation of D-serine in DAO-rich regions as well as the physiological/pathological roles of DAO in white matter astrocytes.
    Frontiers in Synaptic Neuroscience 06/2014; 6:14. DOI:10.3389/fnsyn.2014.00014
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