Decreased levels of d-aspartate and NMDA in the prefrontal cortex and striatum of patients with schizophrenia

Abstract

The potential implication of a decrease in the function of N-methyl-d-aspartate receptors (NMDARs) in the pathophysiology of schizophrenia has long been hypothesised. Accordingly, compounds that inhibit the glycine-1 transporter or target the glycine-binding site of NMDARs, including the co-agonists d-serine and glycine, have shown promise in treating the symptoms of schizophrenia. Clinical interest for d-serine has also been supported by evidence for its abnormal metabolism in schizophrenic patients. Together with d-serine, another d-form amino acid, d-aspartate, exists in the brain of mammals. Synthesised by the enzyme aspartate racemase, d-aspartate is highly concentrated in the prenatal brain; after birth, its levels sharply decrease due to the catabolising activity of the enzyme d-aspartate oxidase. d-aspartate is able to stimulate NMDAR-dependent neurotransmission through direct action at the glutamate-binding site of NMDARs, thus functioning as an endogenous agonist for this subclass of glutamate receptors. In this study, we evaluated for the first time the content of d-aspartate and of its derivative, NMDA, in the post-mortem prefrontal cortex and striatum of schizophrenic patients. Moreover, in the same brain samples, we analysed the expression levels of the subunits that form NMDARs, which are the in vivo targets of d-aspartate and NMDA. Interestingly, we found that d-aspartate and NMDA are consistently decreased in schizophrenia brains compared to control brains. In the prefrontal cortex, this decrease is correlated with a marked downregulation of NMDAR subunits. Overall, these results agree with the innovative therapeutic research in schizophrenia that is aimed at targeting glutamatergic transmission via d-amino acids.

Introduction

Molecular and clinical evidence supports a role for NMDA receptor (NMDAR) hypofunction in the pathogenesis of schizophrenia (SCZ) (Coyle, 2012; Javitt, 2012; Sawa and Snyder, 2003). Accordingly, several clinical trials designed to target the glycine (Gly)-binding site of NMDARs have shown beneficial effects of d-serine (d-Ser), d-cycloserine and glycine (Gly) in treating the clinical symptoms of SCZ (Coyle et al., 2002; Goff et al., 1995; Heresco-Levy et al., 2002; Javitt et al., 1994; Tsai et al., 1998). Likewise, glycine-1 transporter (GlyT1) inhibitors are considered to be a promising tool for the treatment of SCZ (de Bartolomeis et al., 2012; Mohler et al., 2011). Clinical interest in d-Ser has also been supported by its abnormal content and metabolism in the cerebrospinal fluid and serum of SCZ patients (Bendikov et al., 2007; Hashimoto et al., 2003, 2005). Among the few d-amino acids found in the mammalian central nervous system, d-aspartate (d-Asp) together with d-Ser are the most abundant (Hashimoto and Oka, 1997). In contrast to d-Ser, whose presence remains constant during life (Hashimoto et al., 1995), d-Asp undergoes a strict developmental regulation since it is expressed at high levels during embryonic and early phases of life, and then markedly falls during adulthood (Sakai et al., 1998; Wolosker et al., 2000). Remarkably, in the human frontal cortex at 14 weeks of gestation, d-Asp levels even exceed those of the corresponding l-form of this amino acid (Hashimoto et al., 1993). In mammals, d-Asp is catabolised by d-aspartate oxidase (Krebs, 1935; Sacchi et al., 2012) (DDO) and is synthesised by aspartate racemase (Kim et al., 2010) (DR). To date, knowledge about the role of d-Asp in the mammalian brain has been mainly focused on its ability to bind and stimulate NMDARs through its direct binding at the glutamate (Glu) site of this receptor (D'Aniello et al., 2011; Errico et al., 2012; Fagg and Matus, 1984; Ota et al., 2012). Moreover, d-Asp may also indirectly stimulate NMDARs because it represents the substrate for the biosynthesis of endogenous NMDA (D'Aniello et al., 2000a,b). On the basis of the recent interest in d-amino acids in psychiatric disorders, we investigated the levels of d-Asp and its derivative, NMDA, in post-mortem brains of SCZ patients.