Molecular characterization of N-acetylaspartylglutamate synthetase

NAAG synthetase deficiency has only low influence on pathogenesis in a Canavan disease mouse model

Canavan disease (CD) is a leukodystrophy caused by mutations in the N-acetylaspartate (NAA) hydrolase aspartoacylase (ASPA). Inability to degrade NAA and its accumulation in the brain results in spongiform myelin degeneration. NAA is mainly synthesized by neurons, where it is also a precursor of the neuropeptide N-acetylaspartylglutamate (NAAG). Hydrolysis of this peptide by glutamate carboxypeptidases is an additional source of extracellular NAA besides the instant neuronal release of NAA. This study examines to what extent NAA released from NAAG contributes to NAA accumulation and pathogenesis in the brain of Aspanur7/nur7 mutant mice, an established model of CD. Towards this aim, Aspanur7/nur7 mice with additional deficiencies in NAAG synthetase genes Rimklb and/or Rimkla were generated. Loss of myelin in Aspanur7/nur7 mice was not significantly affected by Rimkla and Rimklb deficiency and there was also no obvious change in the extent of brain vacuolation. Astrogliosis was slightly reduced in the forebrain of Rimkla and Rimklb double deficient Aspanur7/nur7 mice. However, only minor improvements at the behavioral level were found. The brain NAA accumulation in CD mice was, however, not significantly reduced in the absence of NAAG synthesis. In summary, there was only a weak tendency towards reduced pathogenic symptoms in Aspanur7/nur7 mice deficient in NAAG synthesis. Therefore, we conclude that NAAG metabolism has little influence on NAA accumulation in Aspanur7/nur7 mice and development of pathological symptoms in CD.

Shati/Nat8l Overexpression Improves Cognitive Decline by Upregulating Neuronal Trophic Factor in Alzheimer's Disease Model Mice

Alzheimer's disease (AD) is a type of dementia characterized by the deposition of amyloid β, a causative protein of AD, in the brain. Shati/Nat8l, identified as a psychiatric disease related molecule, is a responsive enzyme of N-acetylaspartate (NAA) synthesis. In the hippocampi of AD patients and model mice, the NAA content and Shati/Nat8l expression were reported to be reduced. Having recently clarified the involvement of Shati/Nat8l in cognitive function, we examined the recovery effect of the hippocampal overexpression of Shati/Nat8l in AD model mice (5XFAD). Shati/Nat8l overexpression suppressed cognitive dysfunction without affecting the Aβ burden or number of NeuN-positive neurons. In addition, brain-derived neurotrophic factor mRNA was upregulated by Shati/Nat8l overexpression in 5XFAD mice. These results suggest that Shati/Nat8l overexpression prevents cognitive dysfunction in 5XFAD mice, indicating that Shati/Nat8l could be a therapeutic target for AD.

Citrylglutamate synthase deficient male mice are subfertile with impaired histone and transition protein 2 removal in late spermatids

Chromatin remodelling in spermatids is an essential step in spermiogenesis and involves the exchange of most histones by protamines, which drives chromatin condensation in late spermatids. The gene Rimklb encodes a citrylglutamate synthase highly expressed in testes of vertebrates and the increase of its reaction product, β-citrylglutamate, correlates in time with the appearance of spermatids. Here we show that deficiency in a functional Rimklb gene leads to male subfertility, which could be partially rescued by in vitro fertilization. Rimklb-deficient mice are impaired in a late step of spermiogenesis and produce spermatozoa with abnormally shaped heads and nuclei. Sperm chromatin in Rimklb-deficient mice was less condensed and showed impaired histone to protamine exchange and retained transition protein 2. These observations suggest that citrylglutamate synthase, probably via its reaction product β-citrylglutamate, is essential for efficient chromatin remodelling during spermiogenesis and may be a possible candidate gene for male subfertility or infertility in humans.

Increasing Aspartoacylase in the Central Amygdala: The Common Mechanism of Gastroprotective Effects of Monoamine-Based Antidepressants Against Stress

Monoamine-based antidepressants can prophylactically protect against stress-induced gastric ulcers. Although the central nucleus of amygdala (CeA) has been shown to modulate the severity of stress ulcers, little is known about the molecular mechanisms underlying the gastroprotective effect of this kind of drugs. Here, we first used proton magnetic resonance spectroscopy, a non-invasive tool, to explore the change of neurometabolites of the CeA of rats pretreated with the duloxetine of selective serotonin-norepinephrine reuptake inhibitors during 6 h of water-immersion restraint stress (WIRS). Duloxetine decreased N-acetyl-aspartate/creatine ratio (NAA/creatine) in CeA after WIRS, which was paralleled by the amelioration of gastric lesions. Meanwhile, the gastric ulcer index was negatively correlated with reduced NAA/creatine. Furthermore, the intra-CeA infusion of NAA aggravated WIRS-induced gastric mucosa damage, which suggested the crucial role of reduced NAA. Western blotting was performed to identify the specific enzymes responsible for the change of the contents of NAA at 0.5 h/3 h/6 h after WIRS, considering the preventative gastric protection of duloxetine. The NAA-catabolizing enzyme aspartoacylase (ASPA) was the only enzyme downregulated by 0.5 h WIRS and upregulated by duloxetine. Moreover, overexpressing ASPA in CeA alleviated stress ulcers. Additionally, all of the other three monoamine-based antidepressants, the fluoxetine of selective serotonin reuptake inhibitors, the amitriptyline of tricyclic agents, and the moclobemide of MAOs, increased ASPA expression in CeA. Together, these results indicate that increasing ASPA to hydrolyze NAA in CeA is a common mechanism of gastroprotective effects against stress exerted by monoamine-based antidepressants, and ASPA is a shared target more than monoamine regulation for this kind of drugs.

Dendrimer-2PMPA selectively blocks upregulated microglial GCPII activity and improves cognition in a mouse model of multiple sclerosis

Cognitive impairment is a common aspect of multiple sclerosis (MS) for which there are no treatments. Reduced brain N-acetylaspartylglutamate (NAAG) levels are linked to impaired cognition in various neurological diseases, including MS. NAAG levels are regulated by glutamate carboxypeptidase II (GCPII), which hydrolyzes the neuropeptide to N-acetyl-aspartate and glutamate. GCPII activity is upregulated multifold in microglia following neuroinflammation. Although several GCPII inhibitors, such as 2-PMPA, elevate brain NAAG levels and restore cognitive function in preclinical studies when given at high systemic doses or via direct brain injection, none are clinically available due to poor bioavailability and limited brain penetration. Hydroxyl-dendrimers have been successfully used to selectively deliver drugs to activated glia.

Methods: We attached 2-PMPA to hydroxyl polyamidoamine (PAMAM) dendrimers (D-2PMPA) using a click chemistry approach. Cy5-labelled-D-2PMPA was used to visualize selective glial uptake in vitro and in vivo. D-2PMPA was evaluated for anti-inflammatory effects in LPS-treated glial cultures. In experimental autoimmune encephalomyelitis (EAE)-immunized mice, D-2PMPA was dosed biweekly starting at disease onset and cognition was assessed using the Barnes maze, and GCPII activity was measured in CD11b+ hippocampal cells.

Results: D-2PMPA showed preferential uptake into microglia and robust anti-inflammatory activity, including elevations in NAAG, TGFβ, and mGluR3 in glial cultures. D-2PMPA significantly improved cognition in EAE mice, even though physical severity was unaffected. GCPII activity increased >20-fold in CD11b+ cells from EAE mice, which was significantly mitigated by D-2PMPA treatment.

Conclusions: Hydroxyl dendrimers facilitate targeted drug delivery to activated microglia. These data support further development of D-2PMPA to attenuate elevated microglial GCPII activity and treat cognitive impairment in MS.

Alleviation of Methamphetamine Sensitization by Partially Lesioning Dopaminergic Terminals with 6-Hydroxydopamine in Nucleus Accumbens

Amphetamine-type stimulants have become important and popular abused drugs worldwide. Methamphetamine (Meth) sensitization, characterized by a progressive increase in behavioral responses after repeated administration, has been reported in rodents and patients. This behavioral effect has been used as a laboratory model to study drug addiction and schizophrenia. The mesolimbic dopaminergic pathway plays a significant role in the development of Meth behavioral sensitization. Previous studies have reported that the ablation of nucleus accumbens (NAc) by electrolytic or thermal lesioning attenuates addictive behavior to opioids in animals. However, these studies were only conducted in opioid addictive rodents. Furthermore, these ablation procedures also damaged the non-dopaminergic neurons and fibers passing through the NAc. The purpose of this study was to examine the therapeutic effect of NAc lesioning by a selective dopaminergic toxin in Meth-sensitized animals. Adult mice received repeated administration of Meth for 7 days. Open-field locomotor activity and stereotype behavior were significantly increased after Meth treatment, suggesting behavior sensitization. A partial lesion of dopaminergic terminals was made through stereotaxic administration of dopaminergic toxin 6-hydroxydopamine (6-OHDA) to the NAc in the Meth -sensitized mice. Meth behavioral sensitization was significantly antagonized after the lesioning. Brain tissue was collected for qRT-PCR analysis. Repeated administration of Meth increased the expression of tyrosine hydroxylase (TH), BDNF, and Shati, a marker for Meth sensitization, in the NAc. Treatment with 6-OHDA significantly antagonized the upregulation of TH and Shati. Taken together, these data suggest that local administration of 6-OHDA mitigated Meth sensitization in chronic Meth-treated animals. Our data support a new surgical treatment strategy for Meth abuse.

A Role of BDNF in the Depression Pathogenesis and a Potential Target as Antidepressant: The Modulator of Stress Sensitivity "Shati/Nat8l-BDNF System" in the Dorsal Striatum

Depression is one of the most common mental diseases, with increasing numbers of patients globally each year. In addition, approximately 30% of patients with depression are resistant to any treatment and do not show an expected response to first-line antidepressant drugs. Therefore, novel antidepressant agents and strategies are required. Although depression is triggered by post-birth stress, while some individuals show the pathology of depression, others remain resilient. The molecular mechanisms underlying stress sensitivity remain unknown. Brain-derived neurotrophic factor (BDNF) has both pro- and anti-depressant effects, dependent on brain region. Considering the strong region-specific contribution of BDNF to depression pathogenesis, the regulation of BDNF in the whole brain is not a beneficial strategy for the treatment of depression. We reviewed a novel finding of BDNF function in the dorsal striatum, which induces vulnerability to social stress, in addition to recent research progress regarding the brain regional functions of BDNF, including the prefrontal cortex, hippocampus, and nucleus accumbens. Striatal BDNF is regulated by Shati/Nat8l, an N-acetyltransferase through epigenetic regulation. Targeting of Shati/Nat8l would allow BDNF to be striatum-specifically regulated, and the striatal Shati/Nat8l-BDNF pathway could be a promising novel therapeutic agent for the treatment of depression by modulating sensitivity to stress.

Striatal Shati/Nat8l-BDNF pathways determine the sensitivity to social defeat stress in mice through epigenetic regulation

The global number of patients with depression increases in correlation to exposure to social stress. Chronic stress does not trigger depression in all individuals, as some remain resilient. The underlying molecular mechanisms that contribute to stress sensitivity have been poorly understood, although revealing the regulation of stress sensitivity could help develop treatments for depression. We previously found that striatal Shati/Nat8l, an N-acetyltransferase, was increased in a depression mouse model. We investigated the roles of Shati/Nat8l in stress sensitivity in mice and found that Shati/Nat8l and brain-derived neurotrophic factor (BDNF) levels in the dorsal striatum were increased in stress-susceptible mice but not in resilient mice exposed to repeated social defeat stress (RSDS). Knockdown of Shati/Nat8l in the dorsal striatum induced resilience to RSDS. In addition, blockade of BDNF signaling in the dorsal striatum by ANA-12, a BDNF-specific receptor tropomyosin-receptor-kinase B (TrkB) inhibitor, also induced resilience to stress. Shati/Nat8l is correlated with BDNF expression after RSDS, and BDNF is downstream of Shati/Nat8l pathways in the dorsal striatum; Shati/Nat8l is epigenetically regulated by BDNF via histone acetylation. Our results demonstrate that striatal Shati/Nat8l-BDNF pathways determine stress sensitivity through epigenetic regulation. The striatal Shati/Nat8l-BDNF pathway could be a novel target for treatments of depression and could establish a novel therapeutic strategy for depression patients.

Mice deficient in the NAAG synthetase II gene Rimkla are impaired in a novel object recognition task

N-acetylaspartylglutamate (NAAG) is an abundant neuropeptide in the mammalian nervous system, synthesized by two related NAAG synthetases I and II (NAAGS-I and -II) encoded by the genes Rimklb and Rimkla, respectively. NAAG plays a role in cognition and memory, according to studies using inhibitors of the NAAG hydrolase glutamate carboxypeptidase II that increase NAAG concentration. To examine consequences of reduced NAAG concentration, Rimkla-deficient (Rimkla^-/- ) mice were generated. These mice exhibit normal NAAG level at birth, likely because of the intact Rimklb gene, but have significantly reduced NAAG levels in all brain regions in adulthood. In wild type mice NAAGS-II was most abundant in brainstem and spinal cord, as demonstrated using a new NAAGS-II antiserum. In the hippocampus, NAAGS-II was only detectable in neurons expressing parvalbumin, a marker of GABAergic interneurons. Apart from reduced open field activity, general behavior of adult (6 months old) Rimkla^-/- mice examined in different tests (dark-light transition, optokinetic behavior, rotarod, and alternating T-maze) was not significantly altered. However, Rimkla^-/- mice were impaired in a short-term novel object recognition test. This was also the case for mice lacking NAA synthase Nat8l, which are devoid of NAAG. Together with results from previous studies showing that inhibition of the NAAG degrading enzyme glutamate carboxypeptidase II is associated with a significant improvement in object recognition, these results suggest a direct involvement of NAAG synthesized by NAAGS-II in the memory consolidation underlying the novel object recognition task.

N-Acetylaspartyl-Glutamate Metabolism in the Cingulated Cortices as a Biomarker of the Etiology in ASD: A 1H-MRS Model

As brain functional resonance magnetic studies show an aberrant trajectory of neurodevelopment, it is reasonable to predict that the degree of neurochemical abnormalities indexed by magnetic resonance spectroscopy (1H-MRS) might also change according to the developmental stages and brain regions in autism spectrum disorders (ASDs). Since specific N-Acetyl-aspartate (NAA) changes in children's metabolism have been found in the anterior cingulate cortex (ACC) but not in the posterior cingulate cortex (PCC), we analyzed whether the metabolites of ASD youths change between the cingulate cortices using 1H-MRS. l-glutamate (Glu) and l-Acetyl-aspartate (NAA) are products from the N-Acetyl-aspartyl-glutamate (NAAG) metabolism in a reaction that requires the participation of neurons, oligodendrocytes, and astrocytes. This altered tri-cellular metabolism has been described in several neurological diseases, but not in ASD. Compared to the typical development (TD) group, the ASD group had an abnormal pattern of metabolites in the ACC, with a significant increase of glutamate (12.10 ± 3.92 mM; p = 0.02); additionally, N-Acetyl-aspartyl-glutamate significantly decreased (0.41 ± 0.27 mM; p = 0.02) within ASD metabolism abnormalities in the ACC, which may allow the development of new therapeutic possibilities.

Investigating DNA Methylation of SHATI/NAT8L Promoter Sites in Blood of Unmedicated Patients with Major Depressive Disorder

Major depressive disorder (MDD) is one of the most common psychiatric diseases. However, early detection and diagnosis of MDD is difficult, largely because there is no known biomarker or objective diagnostic examination, and its diagnosis is instead based on a clinical interview. The aim of this study was to develop a novel diagnostic tool using DNA methylation as a blood biomarker. We sought to determine whether unmedicated patients with MDD showed significant differences in DNA methylation in the promoter region of the SHATI/N-acetyltransferase 8 like (SHATI/NAT8L) gene compared to healthy controls. Sixty participants with MDD were recruited from all over Japan. They were diagnosed and assessed by at least two trained psychiatrists according to DSM-5 criteria. DNA was extracted from peripheral blood. We then assessed DNA methylation of the SHATI/NAT8L promoter regions in patients with MDD by pyrosequencing. Methylation levels of the SHATI/NAT8L promoter region at CpG sites in peripheral blood from unmedicated patients were significantly higher than in healthy controls. In contrast, medicated patients with MDD showed significantly lower methylation levels in the same region compared to healthy controls. Since previous studies of DNA methylation in MDD only assessed medicated patients, the methylation status of the SHATI/NAT8L promoter region in unmedicated patients presented herein may prove useful for the diagnosis of MDD. To our knowledge, this is the first attempt to measure methylation of the SHATI/NAT8L gene in drug-naïve patients with psychiatric diseases. Based on our findings, methylation of SHATI/NAT8L DNA might be a diagnostic biomarker of MDD.

Association of Missense Mutation in FOLH1 With Decreased NAAG Levels and Impaired Working Memory Circuitry and Cognition

OBJECTIVE

Altering the metabotropic glutamate receptor 3 (mGluR3) by pharmacology or genetics is associated with differences in learning and memory in animals and humans. GRM3 (the gene coding for mGluR3) is also genome-wide associated with risk for schizophrenia. The neurotransmitter N-acetyl-aspartyl-glutamate (NAAG) is the selective endogenous agonist of mGluR3, and increasing NAAG may improve cognition. Glutamate carboxypeptidase II (GCPII), coded by the gene folate hydrolase 1 (FOLH1), regulates the amount of NAAG in the synapse. The goal of this study was to determine the relationship between FOLH1, NAAG levels, measures of human cognition, and neural activity associated with cognition.

METHODS

The effects of genetic variation in FOLH1 on mRNA expression in human brain and NAAG levels using 7-T magnetic resonance spectroscopy (MRS) were measured. NAAG levels and FOLH1 genetic variation were correlated with measures of cognition in subjects with psychosis and unaffected subjects. Additionally, FOLH1 genetic variation was correlated with neural activity during working memory, as measured by functional MRI (fMRI).

RESULTS

A missense mutation in FOLH1 (rs202676 G allele) was associated with increased FOLH1 mRNA in the dorsolateral prefrontal cortex of brains from unaffected subjects and schizophrenia patients. This FOLH1 variant was associated with decreased NAAG levels in unaffected subjects and patients with psychosis. NAAG levels were positively correlated with visual memory performance. Carriers of the FOLH1 variant associated with lower NAAG levels had lower IQ scores. Carriers of this FOLH1 variant had less efficient cortical activity during working memory.

CONCLUSIONS

These data show that higher NAAG levels are associated with better cognition, suggesting that increasing NAAG levels through FOLH1/GCPII inhibition may improve cognition. Additionally, NAAG levels measured by MRS and cortical efficiency during working memory measured by fMRI have the potential to be neuroimaging biomarkers for future clinical trials.

Long reach of the NAAG family tree: An Editorial for "Evidence of NAAG-family tripeptide NAAG2 in the Drosophila nervous system" on page 38

The last common ancestor of humans and fruit flies lived about 800 million years ago, yet both of us have nervous systems that share a number of common important features, for example the use of glutamate as a neurotransmitter. We can now possibly add another common feature to the neural tissue of humans and fruit flies which is that of N-acetylaspartylglutamate (NAAG) peptides. This Editorial highlights an article by Kozik and coworkers in the current issue of the Journal of Neurochemistry, in which the authors report the discovery, in Drosophila melanogaster nervous system, of NAA-glutamylglutamate (NAAG2).

Targeted lipidomics and metabolomics evaluations of cortical neuronal stress in schizophrenia

BACKGROUND

Cortical neuronal dysfunction has been proposed to underlie the psychopathology and cognitive dysfunction of schizophrenia. Previously we have reported altered sphingolipid and N-acylphosphatidylserine (NAPS) metabolism in the frontal cortex in schizophrenia. We continue to expand these investigations to define the biochemical basis for these critical neuropathologies.

METHODS

We undertook a targeted high resolution mass spectrometric analysis to validate our previous reports of elevated sphingolipids and NAPS in the frontal cortex of a new cohort of schizophrenia subjects. Furthermore we expanded these analyses to include ceramides, N-acylphosphatidylethanolamines (NAPE), and N-acylethanolamines (NAE). In the same tissue samples we examined N-acetylaspartylglutamate (NAAG), a modulator of excitatory amino acid transmission, hypothesized to be involved in the pathology of schizophrenia.

RESULTS

We repeated our observations of elevated sulfatides in the frontal cortex in schizophrenia. An in-depth analysis of other sphingolipids revealed decrements in ceramide levels and increased levels of lactosylceramides. NAPS also were found to be augmented in schizophrenia as we previously reported. In addition, levels of NAPES, established biomarkers of neuronal stress, were elevated while their metabolites, NAEs were decreased. With regard to excitatory amino acid neurotransmission, NAAG levels were decreased by 50% while the metabolic precursor, N-acetylaspartate was unaltered.

CONCLUSIONS

Our data support the concept of cortical neuronal dysfunction in schizophrenia as indicated by altered metabolism of structural sphingolipids and NAAG, a modulator of excitatory amino acid neurotransmission.

Vulnerability to depressive behavior induced by overexpression of striatal Shati/Nat8l via the serotonergic neuronal pathway in mice

The number of patients with depressive disorders is increasing. However, the mechanism of depression onsets has not been completely revealed. We previously identified Shati/Nat8l, an N-acetyltransferase, in the brain using an animal model of psychosis. In this study, we revealed the involvement of Shati/Nat8l in the vulnerability to major depression. Shati/Nat8l mRNA was increased only in the striatum of mice, which were exposed to chronic social defeat stress. Shati/Nat8l-overexpressed mice showed impairment in social interaction and sucrose preference after the subthreshold social defeat (microdefeat) stress. These depression-like behaviors were restored by fluvoxamine and LY341495 injection prior to these tests. Furthermore, the intracerebral administration of only fluvoxamine, but not of LY341495, to the dorsal striatum and direct infusion of LY341495 to the dorsal raphe also rescued. Taken together, Shati/Nat8l in the striatum has an important role in the vulnerability to depression onsets by regulating the origin of serotonergic neuronal system via GABAergic projection neuron in the dorsal raphe from the dorsal striatum.

Regulatory system of mGluR group II in the nucleus accumbens for methamphetamine-induced dopamine increase by the medial prefrontal cortex

AIM

We previously reported that methamphetamine (METH)-induced conditioned place preference was attenuated by Shati/Nat8l overexpression in the medial prefrontal cortex (mPFC). Shati/Nat8l overexpression in the mPFC expressed lower levels of both glutamate and dopamine (DA) in the nucleus accumbens (NAc) and attenuated METH-induced DA elevation. We suggested a mechanism in which a decline of glutamate levels in the NAc decreases extracellular DA levels. However, the hypothesis has not confirmed.

METHODS

We conducted a recovery experiments by pre-microinjection of an mGluR group II antagonist, LY341495, into the NAc shell of mPFC-Shati/Nat8l-overexpressed mice followed by METH injection and DA levels measurement by in vivo microdialysis.

RESULTS

Pretreatment with LY341495 was able to restore METH-induced DA increase. Furthermore, mice injected with an adeno-associated virus vector containing GFP (AAV-GFP vector) in the mPFC expressed a colocalization of GFP with DARPP-32 a medium spiny neuron (MSN) marker. Next, co-immunostaining of DARPP-32 and neuronal nitric oxide synthase (nNOS: expressed in a subtype of gamma-Aminobutyric acid (GABA interneurons) in ventral tegmental area (VTA) showed a colocalization of nNOS and DARPP-32.

CONCLUSION

These results provided a proof that Shati/Nat8l attenuation of METH-induced DA increase is mediated by mGluR group II in the NAc. Moreover, immunohistochemical study showed a direct connection of mPFC projection neurons with NAc MSN and a connection of MSN projection neurons with a subtype of GABA interneurons in VTA.

N-Acetyl and Glutamatergic Neurometabolites in Perisylvian Brain Regions of Methamphetamine Users

Background

Methamphetamine induces neuronal N-acetyl-aspartate synthesis in preclinical studies. In a preliminary human proton magnetic resonance spectroscopic imaging investigation, we also observed that N-acetyl-aspartate+N-acetyl-aspartyl-glutamate in right inferior frontal cortex correlated with years of heavy methamphetamine abuse. In the same brain region, glutamate+glutamine is lower in methamphetamine users than in controls and is negatively correlated with depression. N-acetyl and glutamatergic neurochemistries therefore merit further investigation in methamphetamine abuse and the associated mood symptoms.

Methods

Magnetic resonance spectroscopic imaging was used to measure N-acetyl-aspartate+N-acetyl-aspartyl-glutamate and glutamate+glutamine in bilateral inferior frontal cortex and insula, a neighboring perisylvian region affected by methamphetamine, of 45 abstinent methamphetamine-dependent and 45 healthy control participants. Regional neurometabolite levels were tested for group differences and associations with duration of heavy methamphetamine use, depressive symptoms, and state anxiety.

Results

In right inferior frontal cortex, N-acetyl-aspartate+N-acetyl-aspartyl-glutamate correlated with years of heavy methamphetamine use (r = +0.45); glutamate+glutamine was lower in methamphetamine users than in controls (9.3%) and correlated negatively with depressive symptoms (r = -0.44). In left insula, N-acetyl-aspartate+N-acetyl-aspartyl-glutamate was 9.1% higher in methamphetamine users than controls. In right insula, glutamate+glutamine was 12.3% lower in methamphetamine users than controls and correlated negatively with depressive symptoms (r = -0.51) and state anxiety (r = -0.47).

Conclusions

The inferior frontal cortex and insula show methamphetamine-related abnormalities, consistent with prior observations of increased cortical N-acetyl-aspartate in methamphetamine-exposed animal models and associations between cortical glutamate and mood in human methamphetamine users.

Magnetic Resonance Spectroscopy in Schizophrenia: Evidence for Glutamatergic Dysfunction and Impaired Energy Metabolism

In the past couple of decades, major efforts were made to increase reliability of metabolic assessments by magnetic resonance methods. Magnetic resonance spectroscopy (MRS) has been valuable for providing in vivo evidence and investigating biomarkers in neuropsychiatric disorders, namely schizophrenia. Alterations of glutamate and glutamine levels in brains of schizophrenia patients relative to healthy subjects are generally interpreted as markers of glutamatergic dysfunction. However, only a small fraction of MRS-detectable glutamate is involved in neurotransmission. Here we review and discuss brain metabolic processes that involve glutamate and that are likely to be implicated in neuropsychiatric disorders.

Single Session Low Frequency Left Dorsolateral Prefrontal Transcranial Magnetic Stimulation Changes Neurometabolite Relationships in Healthy Humans

Background: Dorsolateral prefrontal cortex (DLPFC) low frequency repetitive transcranial magnetic stimulation (LF-rTMS) has shown promise as a treatment and investigative tool in the medical and research communities. Researchers have made significant progress elucidating DLPFC LF-rTMS effects—primarily in individuals with psychiatric disorders. However, more efforts investigating underlying molecular changes and establishing links to functional and behavioral outcomes in healthy humans are needed.

Objective: We aimed to quantify neuromolecular changes and relate these to functional changes following a single session of DLPFC LF-rTMS in healthy participants.

Methods: Eleven participants received sham-controlled neuronavigated 1 Hz rTMS to the region most activated by a 7-letter Sternberg working memory task (SWMT) within the left DLPFC. We quantified SWMT performance, functional magnetic resonance activation and proton Magnetic resonance spectroscopy (MRS) neurometabolite measure changes before and after stimulation.

Results: A single LF-rTMS session was not sufficient to change DLPFC neurometabolite levels and these changes did not correlate with DLPFC activation changes. Real rTMS, however, significantly altered neurometabolite correlations (compared to sham rTMS), both with baseline levels and between the metabolites themselves. Additionally, real rTMS was associated with diminished reaction time (RT) performance improvements and increased activation within the motor, somatosensory and lateral occipital cortices.

Conclusion: These results show that a single session of LF-rTMS is sufficient to influence metabolite relationships and causes widespread activation in healthy humans. Investigating correlational relationships may provide insight into mechanisms underlying LF-rTMS.

Identification of progesterone receptor membrane component-1 as an interaction partner and possible regulator of fatty acid 2-hydroxylase

The fatty acid 2-hydroxylase (FA2H) is essential for synthesis of 2-hydroxylated fatty acids in myelinating and other cells, and deficiency of this enzyme causes a complicated form of hereditary spastic paraplegia also known as fatty acid hydroxylase-associated neurodegeneration. Despite its important role in sphingolipid metabolism, regulation of FA2H and its interaction with other proteins involved in the same or other metabolic pathways is poorly understood. To identify potential interaction partners of the enzyme, quantitative mass spectrometry using stable isotope labeling of cells was combined with formaldehyde cross-linking and proximity biotinylation, respectively. Besides other enzymes involved in sphingolipid synthesis and intermembrane transfer of ceramide, and putative redox partners of FA2H, progesterone receptor membrane component-1 (PGRMC1) and PGRMC2 were identified as putative interaction partners. These two related heme-binding proteins are known to regulate several cytochrome P450 enzymes. Bimolecular fluorescence complementation experiments confirmed the interaction of FA2H with PGRMC1. Moreover, the PGRMC1 inhibitor AG-205 significantly reduced synthesis of hydroxylated ceramide and glucosylceramide in FA2H-expressing cells. This suggests that PGRMC1 may regulate FA2H activity, possibly through its heme chaperone activity.

N-acetylaspartate (NAA) induces neuronal differentiation of SH-SY5Y neuroblastoma cell line and sensitizes it to chemotherapeutic agents

Neuroblastoma is the most commonly extra-cranial solid tumor of childhood frequently diagnosed. The nervous system-specific metabolite N-acetylaspartate (NAA) is synthesized from aspartate and acetyl-CoA in neurons, it is among the most abundant metabolites present in the central nervous system (CNS) and appears to be involved in many CNS disorders. The functional significance of the high NAA concentration in the brain remains uncertain, but it confers to NAA a unique clinical significance exploited in magnetic resonance spectroscopy. In the current study, we show that treatment of SH-SY5Y neuroblastoma-derived cell line with sub-cytotoxic physiological concentrations of NAA inhibits cell growth. This effect is partly due to enhanced apoptosis, shown by decrease of the anti-apoptotic factors survivin and Bcl-xL, and partly to arrest of the cell-cycle progression, linked to enhanced expression of the cyclin-inhibitors p53, p21^Cip1/Waf1 and p27^Kip1. Moreover, NAA-treated SH-SY5Y cells exhibited morphological changes accompanied with increase of the neurogenic markers TH and MAP2 and down-regulation of the pluripotency markers OCT4 and CXCR4/CD184. Finally, NAA-pre-treated SH-SY5Y cells resulted more sensitive to the cytotoxic effect of the chemotherapeutic drugs Cisplatin and 5-fluorouracil.

To our knowledge, this is the first study demonstrating the neuronal differentiating effects of NAA in neuroblastoma cells. NAA may be a potential preconditioning or adjuvant compound in chemotherapeutic treatment.

Extracellular N-acetylaspartylglutamate released in the nucleus accumbens modulates the pain sensation: Analysis using a microdialysis/mass spectrometry integrated system

Various small molecules act as neurotransmitters and orchestrate neural communication. Growing evidence suggests that not only classical neurotransmitters but also several small molecules, including amino acid derivatives, modulate synaptic transmission. As conditions of acute and chronic pain alter neuronal excitability in the nucleus accumbens, we hypothesized that small molecules released in the nucleus accumbens might play important roles in modulating the pain sensation. However, it is not easy to identify possible pain modulators owing to the absence of a method for comprehensively measuring extracellular small molecules in the brain. In this study, through the use of an emerging metabolomics technique, namely ion chromatography coupled with high-resolution mass spectrometry, we simultaneously analyzed the dynamics of more than 60 small molecules in brain fluids collected by microdialysis, under both the application of pain stimuli and the administration of analgesics. We identified N-acetylaspartylglutamate as a potential pain modulator that is endogenously released in the nucleus accumbens. Infusion of N-acetylaspartylglutamate into the nucleus accumbens significantly attenuated the pain induced by the activation of sensory nerves through optical stimulation. These findings suggest that N-acetylaspartylglutamate released in the nucleus accumbens could modulate pain sensation.

Uncoupling N-acetylaspartate from brain pathology: implications for Canavan disease gene therapy

N-Acetylaspartate (NAA) is the second most abundant organic metabolite in the brain, but its physiological significance remains enigmatic. Toxic NAA accumulation appears to be the key factor for neurological decline in Canavan disease—a fatal neurometabolic disorder caused by deficiency in the NAA-degrading enzyme aspartoacylase. To date clinical outcome of gene replacement therapy for this spongiform leukodystrophy has not met expectations. To identify the target tissue and cells for maximum anticipated treatment benefit, we employed comprehensive phenotyping of novel mouse models to assess cell type-specific consequences of NAA depletion or elevation. We show that NAA-deficiency causes neurological deficits affecting unconscious defensive reactions aimed at protecting the body from external threat. This finding suggests, while NAA reduction is pivotal to treat Canavan disease, abrogating NAA synthesis should be avoided. At the other end of the spectrum, while predicting pathological severity in Canavan disease mice, increased brain NAA levels are not neurotoxic per se. In fact, in transgenic mice overexpressing the NAA synthesising enzyme Nat8l in neurons, supra-physiological NAA levels were uncoupled from neurological deficits. In contrast, elimination of aspartoacylase expression exclusively in oligodendrocytes elicited Canavan disease like pathology. Although conditional aspartoacylase deletion in oligodendrocytes abolished expression in the entire CNS, the remaining aspartoacylase in peripheral organs was sufficient to lower NAA levels, delay disease onset and ameliorate histopathology. However, comparable endpoints of the conditional and complete aspartoacylase knockout indicate that optimal Canavan disease gene replacement therapies should restore aspartoacylase expression in oligodendrocytes. On the basis of these findings we executed an ASPA gene replacement therapy targeting oligodendrocytes in Canavan disease mice resulting in reversal of pre-existing CNS pathology and lasting neurological benefits. This finding signifies the first successful post-symptomatic treatment of a white matter disorder using an adeno-associated virus vector tailored towards oligodendroglial-restricted transgene expression.

Striatal N-Acetylaspartate Synthetase Shati/Nat8l Regulates Depression-Like Behaviors via mGluR3-Mediated Serotonergic Suppression in Mice

BACKGROUND

Several clinical studies have suggested that N-acetylaspartate and N-acetylaspartylglutamate levels in the human brain are associated with various psychiatric disorders, including major depressive disorder. We have previously identified Shati/Nat8l, an N-acetyltransferase, in the brain using an animal model of psychosis. Shati/Nat8l synthesizes N-acetylaspartate from L-aspartate and acetyl-coenzyme A. Further, N-acetylaspartate is converted into N-acetylaspartylglutamate, a neurotransmitter for metabotropic glutamate receptor 3.

METHODS

Because Shati/Nat8l mRNA levels were increased in the dorsal striatum of mice following the exposure to forced swimming stress, Shati/Nat8l was overexpressed in mice by the microinjection of adeno-associated virus vectors containing Shati/Nat8l gene into the dorsal striatum (dS-Shati/Nat8l mice). The dS-Shati/Nat8l mice were further assessed using behavioral and neurochemical tests.

RESULTS

The dS-Shati/Nat8l mice exhibited behavioral despair in the forced swimming and tail suspension tests and social withdrawal in the 3-chamber social interaction test. These depression-like behaviors were attenuated by the administration of a metabotropic glutamate receptor 2/3 antagonist and a selective serotonin reuptake inhibitor. Furthermore, the metabolism of N-acetylaspartate to N-acetylaspartylglutamate was decreased in the dorsal striatum of the dS-Shati/Nat8l mice. This finding corresponded with the increased expression of glutamate carboxypeptidase II, an enzyme that metabolizes N-acetylaspartylglutamate present in the extracellular space. Extracellular serotonin levels were lower in the dorsal striatum of the dS-Shati/Nat8l and normal mice that were repeatedly administered a selective glutamate carboxypeptidase II inhibitor.

CONCLUSIONS

Our findings indicate that the striatal expression of N-acetylaspartate synthetase Shati/Nat8l plays a role in major depressive disorder via the metabotropic glutamate receptor 3-mediated functional control of the serotonergic neuronal system.

Increasing N-acetylaspartate in the Brain during Postnatal Myelination Does Not Cause the CNS Pathologies of Canavan Disease

Canavan disease is caused by mutations in the gene encoding aspartoacylase (ASPA), a deacetylase that catabolizes N-acetylaspartate (NAA). The precise involvement of elevated NAA in the pathogenesis of Canavan disease is an ongoing debate. In the present study, we tested the effects of elevated NAA in the brain during postnatal development. Mice were administered high doses of the hydrophobic methyl ester of NAA (M-NAA) twice daily starting on day 7 after birth. This treatment increased NAA levels in the brain to those observed in the brains of Nur7 mice, an established model of Canavan disease. We evaluated various serological parameters, oxidative stress, inflammatory and neurodegeneration markers and the results showed that there were no pathological alterations in any measure with increased brain NAA levels. We examined oxidative stress markers, malondialdehyde content (indicator of lipid peroxidation), expression of NADPH oxidase and nuclear translocation of the stress-responsive transcription factor nuclear factor (erythroid-derived 2)-like 2 (NRF-2) in brain. We also examined additional pathological markers by immunohistochemistry and the expression of activated caspase-3 and interleukin-6 by Western blot. None of the markers were increased in the brains of M-NAA treated mice, and no vacuoles were observed in any brain region. These results show that ASPA expression prevents the pathologies associated with excessive NAA concentrations in the brain during postnatal myelination. We hypothesize that the pathogenesis of Canavan disease involves not only disrupted NAA metabolism, but also excessive NAA related signaling processes in oligodendrocytes that have not been fully determined and we discuss some of the potential mechanisms.

Methamphetamine induces Shati/Nat8L expression in the mouse nucleus accumbens via CREB- and dopamine D1 receptor-dependent mechanism

Shati/Nat8L significantly increased in the nucleus accumbens (NAc) of mice after repeated methamphetamine (METH) treatment. We reported that Shati/Nat8L overexpression in mouse NAc attenuated METH-induced hyperlocomotion, locomotor sensitization, and conditioned place preference. We recently found that Shati/Nat8L overexpression in NAc regulates the dopaminergic neuronal system via the activation of group II mGluRs by elevated N-acetylaspartylglutamate following N-acetylaspartate increase due to the overexpression. These findings suggest that Shati/Nat8L suppresses METH-induced responses. However, the mechanism by which METH increases the Shati/Nat8L mRNA expression in NAc is unclear. To investigate the regulatory mechanism of Shati/Nat8L mRNA expression, we performed a mouse Shati/Nat8L luciferase assay using PC12 cells. Next, we investigated the response of METH to Shati/Nat8L expression and CREB activity using mouse brain slices of NAc, METH administration to mice, and western blotting for CREB activity of specific dopamine receptor signals in vivo and ex vivo. We found that METH activates CREB binding to the Shati/Nat8L promoter to induce the Shati/Nat8L mRNA expression. Furthermore, the dopamine D1 receptor antagonist SCH23390, but not the dopamine D2 receptor antagonist sulpiride, inhibited the upregulation of Shati/Nat8L and CREB activities in the mouse NAc slices. Thus, the administration of the dopamine D1 receptor agonist SKF38393 increased the Shati/Nat8L mRNA expression in mouse NAc. These results showed that the Shati/Nat8L mRNA was increased by METH-induced CREB pathway via dopamine D1 receptor signaling in mouse NAc. These findings may contribute to development of a clinical tool for METH addiction.

Stimulation-induced transient changes in neuronal activity, blood flow and N-acetylaspartate content in rat prefrontal cortex: a chemogenetic fMRS-BOLD study

Brain activation studies in humans have shown the dynamic nature of neuronal N-acetylaspartate (NAA) and N-acetylaspartylglutamate (NAAG) based on changes in their MRS signals in response to stimulation. These studies demonstrated that upon visual stimulation there was a focal increase in cerebral blood flow (CBF) and a decrease in NAA or in the total of NAA and NAAG signals in the visual cortex, and that these changes were reversed upon cessation of stimulation. In the present study we have developed an animal model in order to explore the relationships between brain stimulation, neuronal activity, CBF and NAA. We use "designer receptor exclusively activated by designer drugs" (DREADDs) technology for site-specific neural activation, a local field potential electrophysiological method for measurement of changes in the rate of neuronal activity, functional MRS for measurement of changes in NAA and a blood oxygenation level-dependent (BOLD) MR technique for evaluating changes in CBF. We show that stimulation of the rat prefrontal cortex using DREADDs results in the following: (i) an increase in level of neuronal activity; (ii) an increase in BOLD and (iii) a decrease in the NAA signal. These findings show for the first time the tightly coupled relationships between stimulation, neuron activity, CBF and NAA dynamics in brain, and also provide the first demonstration of the novel inverse stimulation-NAA phenomenon in an animal model.

The therapeutic and diagnostic potential of the prostate specific membrane antigen/glutamate carboxypeptidase II (PSMA/GCPII) in cancer and neurological disease

Prostate specific membrane antigen (PSMA) otherwise known as glutamate carboxypeptidase II (GCPII) is a membrane bound protein that is highly expressed in prostate cancer and in the neovasculature of a wide variety of tumours including glioblastomas, breast and bladder cancers. This protein is also involved in a variety of neurological diseases including schizophrenia and ALS. In recent years, there has been a surge in the development of both diagnostics and therapeutics that take advantage of the expression and activity of PSMA/GCPII. These include gene therapy, immunotherapy, chemotherapy and radiotherapy. In this review, we discuss the biological roles that PSMA/GCPII plays, both in normal and diseased tissues, and the current therapies exploiting its activity that are at the preclinical stage. We conclude by giving an expert opinion on the future direction of PSMA/GCPII based therapies and diagnostics and hurdles that need to be overcome to make them effective and viable.

The Good and Bad Sides of NAAG

Why has such a small peptide been the source of controversy in neuroscience over the last 5 decades? Is N-acetyl-aspartyl-glutamate (NAAG) a neurotransmitter? Is NAAG located in neuronal tissue or in astrocytes? Is NAAG involved in neuropsychiatric and neurodegenerative disorders? Is NAAG therapeutically beneficial in the treatment of stroke or in initiating cascades of events leading to psychosis? After many years of intense research there is no clear consensus within the scientific community on how NAAG behaves in the brain. One of the major controversies about NAAG is its physiological action at N-methyl-d-aspartate (NMDA) receptors. While some researchers strongly argue that NAAG acts as a weak agonist at NMDA receptors, others have suggested that NAAG could behave as a potent antagonist. Published data from our laboratory demonstrate that the effect of NAAG on NMDA receptors could be influenced by a number of factors including the subcellular localization and subunit composition of NMDA receptors, as well as protons. In this chapter, we will summarize the knowledge of the literature on NAAG, however, we will place emphasis on our recently published data. More specifically, we have reported interesting findings on the effects of NAAG on NMDA receptors at synaptic and extrasynaptic sites using a pharmacological paradigm to distinguish the two populations of NMDA receptors. Additionally, we have evaluated the role of NAAG on GluN2A- and GluN2B-containing NMDA receptors using a HEK293 cell recombinant system. Finally, we have studied the effects of NAAG on GluN2A- and GluN2B-containing NMDA receptors in different extracellular pH conditions. We believe that our findings could potentially resolve some aspects of the debate regarding the role of NAAG at NMDA receptors.

N-Acetylaspartate Synthase Deficiency Corrects the Myelin Phenotype in a Canavan Disease Mouse Model But Does Not Affect Survival Time

Canavan disease (CD) is a severe, lethal leukodystrophy caused by deficiency in aspartoacylase (ASPA), which hydrolyzes N-acetylaspartate (NAA). In the brains of CD patients, NAA accumulates to high millimolar concentrations. The pathology of the disease is characterized by loss of oligodendrocytes and spongy myelin degeneration in the CNS. Whether accumulating NAA, absence of NAA-derived acetate, or absence of any unknown functions of the ASPA enzyme is responsible for the pathology of the disease is not fully understood. We generated ASPA-deficient (Aspa(nur7/nur7)) mice that are also deficient for NAA synthase Nat8L (Nat8L(-/-)/Aspa(nur7/nur7)). These mice have no detectable NAA. Nevertheless, they exhibited normal myelin content, myelin sphingolipid composition, and full reversal of spongy myelin and axonal degeneration. Surprisingly, although pathology was fully reversed, the survival time of the mice was not prolonged. In contrast, Aspa(nur7/nur7) mice with only one intact Nat8L allele accumulated less NAA, developed a less severe pathology, phenotypic improvements, and, importantly, an almost normal survival time. Therefore, inhibition of NAA synthase is a promising therapeutic option for CD. The reduced survival rate of Nat8L(-/-)/Aspa(nur7/nur7) mice, however, indicates that complete inhibition of NAA synthase may bear unforeseeable risks for the patient. Furthermore, we demonstrate that acetate derived from NAA is not essential for myelin lipid synthesis and that loss of NAA-derived acetate does not cause the myelin phenotype of Aspa(nur7/nur7) mice. Our data clearly support the hypothesis that NAA accumulation is the major factor in the development of CD.

Overexpression of Shati/Nat8l, an N-acetyltransferase, in the nucleus accumbens attenuates the response to methamphetamine via activation of group II mGluRs in mice

A novel N-acetyltransferase, Shati/Nat8l, was identified in the nucleus accumbens (NAc) of mice with methamphetamine (METH) treatment. Previously we reported that suppression of Shati/Nat8l enhanced METH-induced behavioral alterations via dopaminergic neuronal regulation. However, the physiological mechanisms of Shati/Nat8l on the dopaminergic system in the brain are unclear. In this study, we injected adeno-associated virus (AAV) vector containing Shati/Nat8l into the NAc or dorsal striatum (dS) of mice, to increase Shati/Nat8l expression. Overexpression of Shati/Nat8l in the NAc, but not in the dS, attenuated METH-induced hyperlocomotion, locomotor sensitization, and conditioned place preference in mice. Moreover, the Shati/Nat8l overexpression in the NAc attenuated the elevation of extracellular dopamine levels induced by METH in in vivo microdialysis experiments. These behavioral and neurochemical alterations due to Shati/Nat8l overexpression in the NAc were inhibited by treatment with selective group II metabotropic glutamate receptor type 2 and 3 (mGluR2/3) antagonist LY341495. In the AAV vector-injected NAc, the tissue contents of both N-acetylaspartate and N-acetylaspartylglutamate (NAAG), endogenous mGluR3 agonist, were elevated. The injection of peptidase inhibitor of NAAG or the perfusion of NAAG itself reduced the basal levels of extracellular dopamine in the NAc of naive mice. These results indicate that Shati/Nat8l in the NAc, but not in the dS, plays an important suppressive role in the behavioral responses to METH by controlling the dopaminergic system via activation of group II mGluRs.

N-Acetylaspartate reductions in brain injury: impact on post-injury neuroenergetics, lipid synthesis, and protein acetylation

N-Acetylaspartate (NAA) is employed as a non-invasive marker for neuronal health using proton magnetic resonance spectroscopy (MRS). This utility is afforded by the fact that NAA is one of the most concentrated brain metabolites and that it produces the largest peak in MRS scans of the healthy human brain. NAA levels in the brain are reduced proportionately to the degree of tissue damage after traumatic brain injury (TBI) and the reductions parallel the reductions in ATP levels. Because NAA is the most concentrated acetylated metabolite in the brain, we have hypothesized that NAA acts in part as an extensive reservoir of acetate for acetyl coenzyme A synthesis. Therefore, the loss of NAA after TBI impairs acetyl coenzyme A dependent functions including energy derivation, lipid synthesis, and protein acetylation reactions in distinct ways in different cell populations. The enzymes involved in synthesizing and metabolizing NAA are predominantly expressed in neurons and oligodendrocytes, respectively, and therefore some proportion of NAA must be transferred between cell types before the acetate can be liberated, converted to acetyl coenzyme A and utilized. Studies have indicated that glucose metabolism in neurons is reduced, but that acetate metabolism in astrocytes is increased following TBI, possibly reflecting an increased role for non-glucose energy sources in response to injury. NAA can provide additional acetate for intercellular metabolite trafficking to maintain acetyl CoA levels after injury. Here we explore changes in NAA, acetate, and acetyl coenzyme A metabolism in response to brain injury.

Vesicular uptake of N-acetylaspartylglutamate is catalysed by sialin (SLC17A5)

NAAG (N-acetylaspartylglutamate) is an abundant neuropeptide in the vertebrate nervous system. It is released from synaptic terminals in a calcium-dependent manner and has been shown to act as an agonist at the type II metabotropic glutamate receptor mGluR3. It has been proposed that NAAG may also be released from axons. So far, however, it has remained unclear how NAAG is transported into synaptic or other vesicles before it is secreted. In the present study, we demonstrate that uptake of NAAG and the related peptide NAAG2 (N-acetylaspartylglutamylglutamate) into vesicles depends on the sialic acid transporter sialin (SLC17A5). This was demonstrated using cell lines expressing a cell surface variant of sialin and by functional reconstitution of sialin in liposomes. NAAG uptake into sialin-containing proteoliposomes was detectable in the presence of an active H+-ATPase or valinomycin, indicating that transport is driven by membrane potential rather than H+ gradient. We also show that sialin is most probably the major and possibly only vesicular transporter for NAAG and NAAG2, because ATP-dependent transport of both peptides was not detectable in vesicles isolated from sialin-deficient mice.

N-acetylaspartate (NAA) and N-acetylaspartylglutamate (NAAG) promote growth and inhibit differentiation of glioma stem-like cells

Metabolic reprogramming is a pathological feature of cancer and a driver of tumor cell transformation. N-Acetylaspartate (NAA) is one of the most abundant amino acid derivatives in the brain and serves as a source of metabolic acetate for oligodendrocyte myelination and protein/histone acetylation or a precursor for the synthesis of the neurotransmitter N-acetylaspartylglutamate (NAAG). NAA and NAAG as well as aspartoacylase (ASPA), the enzyme responsible for NAA degradation, are significantly reduced in glioma tumors, suggesting a possible role for decreased acetate metabolism in tumorigenesis. This study sought to examine the effects of NAA and NAAG on primary tumor-derived glioma stem-like cells (GSCs) from oligodendroglioma as well as proneural and mesenchymal glioblastoma, relative to oligodendrocyte progenitor cells (Oli-Neu). Although the NAA dicarboxylate transporter NaDC3 is primarily thought to be expressed by astrocytes, all cell lines expressed NaDC3 and, thus, are capable of NAA up-take. Treatment with NAA or NAAG significantly increased GSC growth and suppressed differentiation of Oli-Neu cells and proneural GSCs. Interestingly, ASPA was expressed in both the cytosol and nuclei of GSCs and exhibited greatest nuclear immunoreactivity in differentiation-resistant GSCs. Both NAA and NAAG elicited the expression of a novel immunoreactive ASPA species in select GSC nuclei, suggesting differential ASPA regulation in response to these metabolites. Therefore, this study highlights a potential role for nuclear ASPA expression in GSC malignancy and suggests that the use of NAA or NAAG is not an appropriate therapeutic approach to increase acetate bioavailability in glioma. Thus, an alternative acetate source is required.

Immunohistological and electrophysiological evidence that N-acetylaspartylglutamate is a co-transmitter at the vertebrate neuromuscular junction

Immunohistochemical studies previously revealed the presence of the peptide transmitter N-acetylaspartylglutamate (NAAG) in spinal motor neurons, axons and presumptive neuromuscular junctions (NMJs). At synapses in the central nervous system, NAAG has been shown to activate the type 3 metabotropic glutamate receptor (mGluR3) and is inactivated by an extracellular peptidase, glutamate carboxypeptidase II. The present study tested the hypothesis that NAAG meets the criteria for classification as a co-transmitter at the vertebrate NMJ. Confocal microscopy confirmed the presence of NAAG immunoreactivity and extended the resolution of the peptide's location in the lizard (Anolis carolinensis) NMJ. NAAG was localised to a presynaptic region immediately adjacent to postsynaptic acetylcholine receptors. NAAG was depleted by potassium-induced depolarisation and by electrical stimulation of motor axons. The NAAG receptor, mGluR3, was localised to the presynaptic terminal consistent with NAAG's demonstrated role as a regulator of synaptic release at central synapses. In contrast, glutamate receptors, type 2 metabotropic glutamate receptor (mGluR2) and N-methyl-d-aspartate, were closely associated with acetylcholine receptors in the postsynaptic membrane. Glutamate carboxypeptidase II, the NAAG-inactivating enzyme, was identified exclusively in perisynaptic glial cells. This localisation was confirmed by the loss of immunoreactivity when these cells were selectively eliminated. Finally, electrophysiological studies showed that exogenous NAAG inhibited evoked neurotransmitter release by activating a group II metabotropic glutamate receptor (mGluR2 or mGluR3). Collectively, these data support the conclusion that NAAG is a co-transmitter at the vertebrate NMJ.

Glutamate carboxypeptidase II in diagnosis and treatment of neurologic disorders and prostate cancer

Glutamate carboxypeptidase II (GCPII) is a membrane-bound binuclear zinc metallopeptidase with the highest expression levels found in the nervous and prostatic tissue. Throughout the nervous system, glia-bound GCPII is intimately involved in the neuron-neuron and neuron-glia signaling via the hydrolysis of N-acetylaspartylglutamate (NAAG), the most abundant mammalian peptidic neurotransmitter. The inhibition of the GCPII-controlled NAAG catabolism has been shown to attenuate neurotoxicity associated with enhanced glutamate transmission and GCPII-specific inhibitors demonstrate efficacy in multiple preclinical models including traumatic brain injury, stroke, neuropathic and inflammatory pain, amyotrophic lateral sclerosis, and schizophrenia. The second major area of pharmacological interventions targeting GCPII focuses on prostate carcinoma; GCPII expression levels are highly increased in androgen-independent and metastatic disease. Consequently, the enzyme serves as a potential target for imaging and therapy. This review offers a summary of GCPII structure, physiological functions in healthy tissues, and its association with various pathologies. The review also outlines the development of GCPII-specific small-molecule compounds and their use in preclinical and clinical settings.

Group II mGluR agonist LY354740 and NAAG peptidase inhibitor effects on prepulse inhibition in PCP and D-amphetamine models of schizophrenia

RATIONALE

Group II metabotropic glutamate receptor (mGluR) agonists represent a novel approach to the treatment of schizophrenia. Inasmuch as the peptide neurotransmitter N-acetylaspartylglutamate (NAAG) activates these receptors, NAAG peptidase inhibitors conceptually represent a parallel path toward development of new antipsychotic drugs. While group II agonists are effective in several animal models of schizophrenia, they are reported to lack efficacy in moderating the effects of phencyclidine (PCP) on prepulse inhibition of acoustic startle in animal models of sensory processing deficits found in this disorder.

OBJECTIVE

The objective of this study was to re-examine the efficacy of a group II metabotropic glutamate agonist and NAAG peptidase inhibitors in prepulse inhibition models of schizophrenia across two strains of mice.

METHODS

The method used was an assay to determine the efficacy of these drugs in moderating the reduction in prepulse inhibition of acoustic startle in mice treated with PCP and D: -amphetamine.

RESULTS

The group II agonist LY354740 (5 and 10 mg/kg) moderated the effects of PCP on prepulse inhibition of acoustic startle in DBA/2 but not C57BL/6 mice. In contrast, two NAAG peptidase inhibitors, ZJ43 (150 mg/kg) and 2-PMPA (50, 100, and 150 mg/kg), did not significantly affect the PCP-induced reduction in prepulse inhibition in either strain.

CONCLUSIONS

These data demonstrate that the efficacy of group II agonists in this model of sensory motor processing is strain-specific in mice. The difference between the effects of the group II agonist and the peptidase inhibitors in the DBA/2 mice may relate to the difference in efficacy of NAAG and the agonist at mGluR2.

Advances in understanding the peptide neurotransmitter NAAG and appearance of a new member of the NAAG neuropeptide family

A substantial body of data was reported between 1984 and 2000 demonstrating that the neuropeptide N-acetylaspartylglutamate (NAAG) not only functions as a neurotransmitter but also is the third most prevalent transmitter in the mammalian nervous system behind glutamate and GABA. By 2005, this conclusion was validated further through a series of studies in vivo and in vitro. The primary enzyme responsible for the inactivation of NAAG following its synaptic release had been cloned, characterized and knocked out. Potent inhibitors of this enzyme were developed and their efficacy has been extensively studied in a series of animal models of clinical conditions, including stroke, peripheral neuropathy, traumatic brain injury, inflammatory and neuropathic pain, cocaine addiction, and schizophrenia. Considerable progress also has been made in defining further the mechanism of action of these peptidase inhibitors in elevating synaptic levels of NAAG with the consequent inhibition of transmitter release via the activation of pre-synaptic metabotropic glutamate receptor 3 by this peptide. Very recent discoveries include identification of two different nervous system enzymes that mediate the synthesis of NAAG from N-acetylaspartate and glutamate and the finding that one of these enzymes also mediates the synthesis of a second member of the NAAG family of neuropeptides, N-acetylaspartylglutamylglutamate.

N-acetylaspartylglutamate synthetase II synthesizes N-acetylaspartylglutamylglutamate

N-Acetylaspartylglutamate (NAAG) is found at high concentrations in the vertebrate nervous system. NAAG is an agonist at group II metabotropic glutamate receptors. In addition to its role as a neuropeptide, a number of functions have been proposed for NAAG, including a role as a non-excitotoxic transport form of glutamate and a molecular water pump. We recently identified a NAAG synthetase (now renamed NAAG synthetase I, NAAGS-I), encoded by the ribosomal modification protein rimK-like family member B (Rimklb) gene, as a member of the ATP-grasp protein family. We show here that a structurally related protein, encoded by the ribosomal modification protein rimK-like family member A (Rimkla) gene, is another NAAG synthetase (NAAGS-II), which in addition, synthesizes the N-acetylated tripeptide N-acetylaspartylglutamylglutamate (NAAG(2)). In contrast, NAAG(2) synthetase activity was undetectable in cells expressing NAAGS-I. Furthermore, we demonstrate by mass spectrometry the presence of NAAG(2) in murine brain tissue and sciatic nerves. The highest concentrations of both, NAAG(2) and NAAG, were found in sciatic nerves, spinal cord, and the brain stem, in accordance with the expression level of NAAGS-II. To our knowledge the presence of NAAG(2) in the vertebrate nervous system has not been described before. The physiological role of NAAG(2), e.g. whether it acts as a neurotransmitter, remains to be determined.

Profiling the effects of isocitrate dehydrogenase 1 and 2 mutations on the cellular metabolome

Point mutations of the NADP(+)-dependent isocitrate dehydrogenases 1 and 2 (IDH1 and IDH2) occur early in the pathogenesis of gliomas. When mutated, IDH1 and IDH2 gain the ability to produce the metabolite (R)-2-hydroxyglutarate (2HG), but the downstream effects of mutant IDH1 and IDH2 proteins or of 2HG on cellular metabolism are unknown. We profiled >200 metabolites in human oligodendroglioma (HOG) cells to determine the effects of expression of IDH1 and IDH2 mutants. Levels of amino acids, glutathione metabolites, choline derivatives, and tricarboxylic acid (TCA) cycle intermediates were altered in mutant IDH1- and IDH2-expressing cells. These changes were similar to those identified after treatment of the cells with 2HG. Remarkably, N-acetyl-aspartyl-glutamate (NAAG), a common dipeptide in brain, was 50-fold reduced in cells expressing IDH1 mutants and 8.3-fold reduced in cells expressing IDH2 mutants. NAAG also was significantly lower in human glioma tissues containing IDH mutations than in gliomas without such mutations. These metabolic changes provide clues to the pathogenesis of tumors associated with IDH gene mutations.