Decreased free d-aspartate levels are linked to enhanced d-aspartate oxidase activity in the dorsolateral prefrontal cortex of schizophrenia patients

Abstract

It is long acknowledged that the N-methyl d-aspartate receptor co-agonist, d-serine, plays a crucial role in several N-methyl d-aspartate receptor-mediated physiological and pathological processes, including schizophrenia. Besides d-serine, another free d-amino acid, d-aspartate, is involved in the activation of N-methyl d-aspartate receptors acting as an agonist of this receptor subclass, and is abundantly detected in the developing human brain. Based on the hypothesis of N-methyl d-aspartate receptor hypofunction in the pathophysiology of schizophrenia and considering the ability of d-aspartate and d-serine to stimulate N-methyl d-aspartate receptor-dependent transmission, in the present work we assessed the concentration of these two d-amino acids in the post-mortem dorsolateral prefrontal cortex and hippocampus of patients with schizophrenia and healthy subjects. Moreover, in this cohort of post-mortem brain samples we investigated the spatiotemporal variations of d-aspartate and d-serine. Consistent with previous work, we found that d-aspartate content was selectively decreased by around 30% in the dorsolateral prefrontal cortex, but not in the hippocampus, of schizophrenia-affected patients, compared to healthy subjects. Interestingly, such selective reduction was associated to greater (around 25%) cortical activity of the enzyme responsible for d-aspartate catabolism, d-aspartate oxidase. Conversely, no significant changes were found in the methylation state and transcription of DDO gene in patients with schizophrenia, compared to control individuals, as well as in the expression levels of serine racemase, the major enzyme responsible for d-serine biosynthesis, which also catalyzes aspartate racemization. These results reveal the potential involvement of altered d-aspartate metabolism in the dorsolateral prefrontal cortex as a factor contributing to dysfunctional N-methyl d-aspartate receptor-mediated transmission in schizophrenia.

Olanzapine, but not clozapine, increases glutamate release in the prefrontal cortex of freely moving mice by inhibiting D-aspartate oxidase activity

“…On the other hand, the reason by which clozapine significantly reduces the cortical L-Asp release in treated animals remains unknown and requires future detailed investigations. Taken together, the present data suggest that the increased DDO activity and the resulting decrease in D-Asp levels recently found in the post-mortem brain of patients with schizophrenia18 are not an epiphenomenon of pharmacological treatment, at least in the context of the antipsychotics used here. Remarkably, for the first time, our observations put forward a translational value for D-Asp metabolism since they highlight an unexpected instrumental role for DDO activity in the modulation of glutamatergic effects of olanzapine.…”

“…However, since DDO has intracellular localization and is active within peroxisomes13, future studies are needed to clarify whether and how olanzapine has direct access to DDO in vivo . Regardless of the precise mechanism responsible for DDO inhibition, the potential relevance of DDO as a pharmacological target in schizophrenia shown by this and previous finding18 prompts to the discovery of novel compounds with inhibitory activity for this enzyme, as recently reported by Homma and co-workers48. In agreement with its agonistic profile not only at NMDARs but also at mGlu5 receptors49, previous observations showed that increased levels of D-Asp in preclinical models are associated with improved memory679, enhanced structural and functional synaptic plasticity11 and greater connectivity in cortico-hippocampal regions16.…”

“…Based on the evidence that DDO plays a pivotal role in the regulation of D-Asp content in the mammalian brain13 and considering the reduction of the activity of this enzyme detected in patients with schizophrenia18, here we screened the potential ability of wide spectrum drugs with psychiatric relevance to affect the human DDO (hDDO) activity. To this aim, we performed in vitro inhibition assays on recombinant hDDO enzyme, using first-generation (chlorpromazine and haloperidol) and second-generation (clozapine and olanzapine) antipsychotics, and antidepressants (amitriptyline, bupropion and fluoxetine) (Table 1).…”

“…In addition to studies in animal models, clinical investigation has shown that a single-nucleotide polymorphism in the DDO gene (rs3757351), predicting reduced expression of DDO mRNA in the post-mortem PFC, is associated in healthy humans to greater prefrontal gray matter and activity during working memory, as detected by fMRI11. Moreover, two independent studies recently showed a significant reduction (~30–40%) of total free D-Asp content in the post-mortem brain of schizophrenia-affected patients1718. In particular, Nuzzo et al .…”

“…In particular, Nuzzo et al . reported that lower levels of D-Asp in the dorsolateral PFC (DLPFC) of patients with schizophrenia are associated to significantly increased DDO enzymatic activity, thus suggesting alterations in D-Asp metabolism in this psychiatric disorder18.…”