Differential distribution of N-acetylaspartylglutamate and N-acetylaspartate immunoreactivities in rat forebrain

Cellular and molecular mechanisms of aspartoacylase and its role in Canavan disease

Canavan disease is an autosomal recessive and lethal neurological disorder, characterized by the spongy degeneration of the white matter in the brain. The disease is caused by a deficiency of the cytosolic aspartoacylase (ASPA) enzyme, which catalyzes the hydrolysis of N-acetyl-aspartate (NAA), an abundant brain metabolite, into aspartate and acetate. On the physiological level, the mechanism of pathogenicity remains somewhat obscure, with multiple, not mutually exclusive, suggested hypotheses. At the molecular level, recent studies have shown that most disease linked ASPA gene variants lead to a structural destabilization and subsequent proteasomal degradation of the ASPA protein variants, and accordingly Canavan disease should in general be considered a protein misfolding disorder. Here, we comprehensively summarize the molecular and cell biology of ASPA, with a particular focus on disease-linked gene variants and the pathophysiology of Canavan disease. We highlight the importance of high-throughput technologies and computational prediction tools for making genotype-phenotype predictions as we await the results of ongoing trials with gene therapy for Canavan disease.

Neurochemistry of the mammillary body

The mammillary body (MB) is a component of the extended hippocampal system and many studies have shown that its functions are vital for mnemonic processes. Together with other subcortical structures, such as the anterior thalamic nuclei and tegmental nuclei of Gudden, the MB plays a crucial role in the processing of spatial and working memory, as well as navigation in rats. The aim of this paper is to review the distribution of various substances in the MB of the rat, with a description of their possible physiological roles. The following groups of substances are reviewed: (1) classical neurotransmitters (glutamate and other excitatory transmitters, gamma-aminobutyric acid, acetylcholine, serotonin, and dopamine), (2) neuropeptides (enkephalins, substance P, cocaine- and amphetamine-regulated transcript, neurotensin, neuropeptide Y, somatostatin, orexins, and galanin), and (3) other substances (calcium-binding proteins and calcium sensor proteins). This detailed description of the chemical parcellation may facilitate a better understanding of the MB functions and its complex relations with other structures of the extended hippocampal system.

Altered prefrontal neurochemistry in the DJ-1 knockout mouse model of Parkinson's disease: complementary semi-quantitative analyses with in vivo magnetic resonance spectroscopy and MALDI-MSI

In vivo proton magnetic resonance spectroscopy (¹H-MRS) and matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI) are two semi-quantitative analytical methods commonly used in neurochemical research. In this study, the two methods were used complementarily, in parallel, to investigate neurochemical perturbations in the medial prefrontal cortex (mPFC) of 9-month-old DJ-1 knockout mice, a well-established transgenic model for Parkinson's diseases. Convergingly, the results obtained with the two methods demonstrated that, compared with the wild-type (WT) mice, the DJ-1 knockout mice had significantly increased glutathione (GSH) level and GSH/glutamate (Glu) ratio in the mPFC, which likely presented an astrocytic compensatory mechanism in response to elevated regional oxidative stress induced by the loss of DJ-1 function. The results from this study also highlighted (1) the need to be cautious when interpreting the in vivo ¹H-MRS results obtained from aged transgenic animals, in which the concentration of internal reference, being whether water or total creatine, could no longer be assumed to be the same as that in the age-matched WT animals, and (2) the necessity and importance of complementary analyses with more than one method under such circumstances.

AGC1 Deficiency: Pathology and Molecular and Cellular Mechanisms of the Disease

AGC1/Aralar/Slc25a12 is the mitochondrial carrier of aspartate-glutamate, the regulatory component of the NADH malate-aspartate shuttle (MAS) that transfers cytosolic redox power to neuronal mitochondria. The deficiency in AGC1/Aralar leads to the human rare disease named "early infantile epileptic encephalopathy 39" (EIEE 39, OMIM # 612949) characterized by epilepsy, hypotonia, arrested psychomotor neurodevelopment, hypo myelination and a drastic drop in brain aspartate (Asp) and N-acetylaspartate (NAA). Current evidence suggest that neurons are the main brain cell type expressing Aralar. However, paradoxically, glial functions such as myelin and Glutamine (Gln) synthesis are markedly impaired in AGC1 deficiency. Herein, we discuss the role of the AGC1/Aralar-MAS pathway in neuronal functions such as Asp and NAA synthesis, lactate use, respiration on glucose, glutamate (Glu) oxidation and other neurometabolic aspects. The possible mechanism triggering the pathophysiological findings in AGC1 deficiency, such as epilepsy and postnatal hypomyelination observed in humans and mice, are also included. Many of these mechanisms arise from findings in the aralar-KO mice model that extensively recapitulate the human disease including the astroglial failure to synthesize Gln and the dopamine (DA) mishandling in the nigrostriatal system. Epilepsy and DA mishandling are a direct consequence of the metabolic defect in neurons due to AGC1/Aralar deficiency. However, the deficits in myelin and Gln synthesis may be a consequence of neuronal affectation or a direct effect of AGC1/Aralar deficiency in glial cells. Further research is needed to clarify this question and delineate the transcellular metabolic fluxes that control brain functions. Finally, we discuss therapeutic approaches successfully used in AGC1-deficient patients and mice.

Met carriers of the BDNF Val66Met polymorphism show reduced Glx/NAA in the pregenual ACC in two independent cohorts

The Met allele of the Val66Met SNP of the BDNF gene (rs6265) is associated with impaired activity-dependent release of brain-derived neurotrophic factor (BDNF), resulting in reduced synaptic plasticity, impaired glutamatergic neurotransmission, and morphological changes. While previous work has demonstrated Val66Met effects on magnetic resonance spectroscopy (MRS) markers of either glutamatergic metabolism (Glx) or neuronal integrity (NAA), no study has investigated Val66Met effects on these related processes simultaneously. As these metabolites share a metabolic pathway, the Glx/NAA ratio may be a more sensitive marker of changes associated with the Val66Met SNP. This ratio is increased in psychiatric disorders linked to decreased functioning in the anterior cingulate cortex (ACC). In this study, we investigated the correlation of the Val66Met polymorphism of the BDNF gene with Glx/NAA in the pregenual anterior cingulate cortex (pgACC) using MRS at 3 Tesla (T) (n = 30, all males) and 7 T (n = 98, 40 females). In both cohorts, Met carriers had lower Glx/NAA compared to Val homozygotes. Follow-up analyses using absolute quantification revealed that the Met carriers do not show decreased pgACC glutamate or glutamine levels, but instead show increased NAA compared to the Val homozygotes. This finding may in part explain conflicting evidence for Val66Met as a risk factor for developing psychiatric illnesses.

Acetate Revisited: A Key Biomolecule at the Nexus of Metabolism, Epigenetics and Oncogenesis-Part 1: Acetyl-CoA, Acetogenesis and Acyl-CoA Short-Chain Synthetases

Acetate is a major end product of bacterial fermentation of fiber in the gut. Acetate, whether derived from the diet or from fermentation in the colon, has been implicated in a range of health benefits. Acetate is also generated in and released from various tissues including the intestine and liver, and is generated within all cells by deacetylation reactions. To be utilized, all acetate, regardless of the source, must be converted to acetyl coenzyme A (acetyl-CoA), which is carried out by enzymes known as acyl-CoA short-chain synthetases. Acyl-CoA short-chain synthetase-2 (ACSS2) is present in the cytosol and nuclei of many cell types, whereas ACSS1 is mitochondrial, with greatest expression in heart, skeletal muscle, and brown adipose tissue. In addition to acting to redistribute carbon systemically like a ketone body, acetate is becoming recognized as a cellular regulatory molecule with diverse functions beyond the formation of acetyl-CoA for energy derivation and lipogenesis. Acetate acts, in part, as a metabolic sensor linking nutrient balance and cellular stress responses with gene transcription and the regulation of protein function. ACSS2 is an important task-switching component of this sensory system wherein nutrient deprivation, hypoxia and other stressors shift ACSS2 from a lipogenic role in the cytoplasm to a regulatory role in the cell nucleus. Protein acetylation is a critical post-translational modification involved in regulating cell behavior, and alterations in protein acetylation status have been linked to multiple disease states, including cancer. Improving our fundamental understanding of the "acetylome" and how acetate is generated and utilized at the subcellular level in different cell types will provide much needed insight into normal and neoplastic cellular metabolism and the epigenetic regulation of phenotypic expression under different physiological stressors. This article is Part 1 of 2 - for Part 2 see doi: 10.3389/fphys.2020.580171.

Cell-Type-Specific Metabolic Profiling Achieved by Combining Desorption Electrospray Ionization Mass Spectrometry Imaging and Immunofluorescence Staining

Cell-type-specific metabolic profiling in tissue with heterogeneous composition has been of great interest across all mass spectrometry imaging (MSI) technologies. We report here a powerful new chemical imaging capability in desorption electrospray ionization (DESI) MSI, which enables cell-type-specific and in situ metabolic profiling in complex tissue samples. We accomplish this by combining DESI-MSI with immunofluorescence staining using specific cell-type markers. We take advantage of the variable frequency of each distinct cell type in the lateral septal nucleus (LSN) region of mouse forebrain. This allows computational deconvolution of the cell-type-specific metabolic profile in neurons and astrocytes by convex optimization-a machine learning method. Based on our approach, we observed 107 metabolites that show different distributions and intensities between astrocytes and neurons. We subsequently identified 23 metabolites using high-resolution mass spectrometry (MS) and tandem MS, which include small metabolites such as adenosine and N-acetylaspartate previously associated with astrocytes and neurons, respectively, as well as accumulation of several phospholipid species in neurons which have not been studied before. Overall, this method overcomes the relatively low spatial resolution of DESI-MSI and provides a new platform for in situ metabolic investigation at the cell-type level in complex tissue samples with heterogeneous cell-type composition.

Interictal Single-Voxel Proton Magnetic Resonance Spectroscopy of the Temporal Lobe in Dogs With Idiopathic Epilepsy

Proton magnetic resonance spectroscopy (H1-MRS) could provide insight into the metabolic pathophysiology of the temporal lobe of canine brain after seizure. Currently, there is no evidence-based data available on MRS of temporal lobe in dogs with idiopathic epilepsy (IE). The aim of this prospective, cross-sectional study was to evaluate the interictal metabolic activity of the temporal lobe in IE dogs compared to a control group with the use of H1-MRS. Ten healthy dogs and 27 client-owned dogs with IE underwent 1.5-Tesla magnetic resonance imaging (MRI) and single-voxel H1-MRS. The MRS studies were acquired as spin echoes with a repetition time (TR) of 2,000 ms and an echo time (TE) of 144 ms. A cubic voxel (10 ×10 ×10 mm) was positioned bilaterally into the region of the left and right temporal lobe, including a middle part of the hippocampus and the amygdala. The N-acetylaspartate (NAA)-to-creatine (NAA/Cr), NAA-to-choline (NAA/Cho), choline-to-creatine (Cho/Cr), and choline-to-NAA (Cho/NAA) ratios were determined in both hemispheres and compared to controls. No significant differences in all metabolite ratios between epileptic dogs and the control group could be found. A time-dependent decrease in the NAA/Cho ratio as well as an increase in the Cho/NAA ratio was found with proximity in time to the last seizure. We found no correlation between metabolite ratios and age or sex in this animal group. Time span from the last seizure to the acquisition of MRS significantly correlated with NAA/Cho and Cho/NAA ratio. We conclude that without a time relation, metabolite ratios in dogs with IE do not differ from those of the control group.

Serum copper and zinc levels correlate with biochemical metabolite ratios in the prefrontal cortex and lentiform nucleus of patients with major depressive disorder

BACKGROUND

Previous studies have demonstrated that copper and zinc metabolism are associated with the development of major depressive disorder (MDD). Abnormal copper and zinc levels may be related to neurotransmission and biochemical metabolism in the brains of MDD patients, especially in the prefrontal cortex (PFC) and lentiform nucleus (LN). However, the mechanism of how copper and zinc levels contribute to neural metabolism in MDD patients remains to be deciphered. This study aimed to correlate copper and zinc levels with biochemical metabolite ratios in the PFC and LN of MDD patients.

METHOD

Twenty-nine MDD patients and thirty-two healthy control (HC) volunteers were enrolled in this study. Proton magnetic resonance spectroscopy (¹H-MRS) was used to determine the levels of the N-acetylaspartate (NAA), choline (Cho) and creatine (Cr) in the brain, and specifically in the PFC and LN regions. Serum copper and zinc levels were measured using atomic emission spectrometry (AES). Afterwards, copper and zinc levels were correlated with biochemical metabolite ratios in the PFC and LN regions of the brain.

RESULTS

Higher serum copper and lower serum zinc levels with higher copper/zinc ratios were observed in MDD patients. NAA/Cr ratios in the PFC of MDD patients were lower compared to HC volunteers. In MDD patients, serum copper levels were negatively correlated with NAA/Cr ratios in the right PFC and right LN, while copper/zinc ratios were negatively correlated with NAA/Cr ratios in the right LN. No significant differences in serum copper and zinc levels with NAA/Cr ratios in the left PFC and left LN were observed in MDD patients.

CONCLUSION

Our findings suggest that higher serum copper and lower serum zinc levels may contribute to neuronal impairment by affecting neuronal biochemical metabolite ratios in the right PFC and right LN of MDD patients. Abnormal copper and zinc levels may play an important role in the pathophysiology of MDD.

Generation of an Atxn2-CAG100 knock-in mouse reveals N-acetylaspartate production deficit due to early Nat8l dysregulation

Spinocerebellar ataxia type 2 (SCA2) is an autosomal dominant neurodegenerative disorder caused by CAG-expansion mutations in the ATXN2 gene, mainly affecting motor neurons in the spinal cord and Purkinje neurons in the cerebellum. While the large expansions were shown to cause SCA2, the intermediate length expansions lead to increased risk for several atrophic processes including amyotrophic lateral sclerosis and Parkinson variants, e.g. progressive supranuclear palsy. Intense efforts to pioneer a neuroprotective therapy for SCA2 require longitudinal monitoring of patients and identification of crucial molecular pathways. The ataxin-2 (ATXN2) protein is mainly involved in RNA translation control and regulation of nutrient metabolism during stress periods. The preferential mRNA targets of ATXN2 are yet to be determined. In order to understand the molecular disease mechanism throughout different prognostic stages, we generated an Atxn2-CAG100-knock-in (KIN) mouse model of SCA2 with intact murine ATXN2 expression regulation. Its characterization revealed somatic mosaicism of the expansion, with shortened lifespan, a progressive spatio-temporal pattern of pathology with subsequent phenotypes, and anomalies of brain metabolites such as N-acetylaspartate (NAA), all of which mirror faithfully the findings in SCA2 patients. Novel molecular analyses from stages before the onset of motor deficits revealed a strong selective effect of ATXN2 on Nat8l mRNA which encodes the enzyme responsible for NAA synthesis. This metabolite is a prominent energy store of the brain and a well-established marker for neuronal health. Overall, we present a novel authentic rodent model of SCA2, where in vivo magnetic resonance imaging was feasible to monitor progression and where the definition of earliest transcriptional abnormalities was possible. We believe that this model will not only reveal crucial insights regarding the pathomechanism of SCA2 and other ATXN2-associated disorders, but will also aid in developing gene-targeted therapies and disease prevention.

Regional metabolic differences in rat prefrontal cortex measured with in vivo 1 H-MRS correlate with regional histochemical differences

Many neurological/psychiatric disorders are associated with metabolic abnormalities in the brain observable with in vivo proton MRS (¹ H-MRS). The underlying molecular/cellular mechanisms and functional correlations of such metabolic alterations, however, are yet to be understood fully. The rodent prefrontal cortex (PFC) is comprised of multiple sub-regions with distinctive cytoarchitecture and functions, providing a good model system to study the correlations among cerebral metabolism, regional cytoarchitecture and connectivity. In this study, the metabolic profiles in two voxels containing mainly the medial PFC (mPFC) and posterior part of the cingulate cortex (pCG), respectively, were measured with single-voxel in vivo ¹ H-MRS in adult male rats. The levels of glutamine synthetase and glutamatergic synaptic proteins, including vesicular glutamate transporter 1, vesicular glutamate transporter 2 (VGLUT2) and post-synaptic density protein 95 (PSD95), as well as the density of astrocytes, in these two regions were also compared semi-quantitatively. It was shown that, relative to the pCG voxel, the mPFC voxel had significantly higher N-acetyl aspartate, glutamate (Glu), glutamine (Gln), Glx (Glu + Gln), myo-inositol and taurine levels. The VGLUT2/PSD95 levels and astrocyte density were also higher in the mPFC voxel than in the pCG voxel. Taken together, these results indicated that regional metabolic variations in the PFC of the adult male rat may reflect regional differences in the density of astrocytes and glutamatergic terminals associated with subcortical projections. The study provided a link between the Glu concentration measured with localized in vivo ¹ H-MRS and regional glutamatergic activities/connections in the rat PFC.

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.

A pilot study of hippocampal N-acetyl-aspartate in youth with treatment resistant major depression

BACKGROUND

Smaller hippocampal volumes, as assessed by magnetic resonance imaging (MRI), and proton magnetic resonance spectroscopy (¹H-MRS) indexed alterations in brain metabolites have been identified in adults with major depressive disorder (MDD). Our group has found similar effects in MDD youth. However, this has not been studied in youth with treatment resistant MDD (TRD), nor has the interaction between regional N-acetyl-aspartate and volume deficits. N-acetyl-aspartate is an amino acid in the synthesis pathway of glutamate, and serves a marker of neuronal viability/number.

METHODS

Fifteen typically developing youth (16-22 years of age; 7 males, 8 females) and eighteen youth with TRD (14-22 years of age; 8 males, 10 females) underwent ¹H-MRS and MRI on a 3T scanner. A short echo PRESS protocol was used with voxels in the right and left hippocampi (6mL each). Hippocampal volume was evaluated using FreeSurfer.

RESULTS

Compared with the typically developing group, youth with TRD had lower concentrations of N-acetyl-aspartate in the left hippocampus (p=0.004), and a trend for smaller left hippocampal volume (p=0.067). In TRD subjects, hippocampal N-acetyl-aspartate was inversely correlated with left (r=-0.68, p=0.003) but not right hippocampal volume. Right hippocampal glutamate+glutamine was greater in TRD youth compared to typically developing controls (p=0.007).

CONCLUSIONS

These results suggest a neurochemical and structural deficit in the hippocampi of youth with TRD. These findings fit with the role of N-acetyl-aspartate in glutamate neurotransmission and the effect of glutamate on brain morphology.

A role for the locus coeruleus in the analgesic efficacy of N-acetylaspartylglutamate peptidase (GCPII) inhibitors ZJ43 and 2-PMPA

N-acetylaspartylglutamate (NAAG) is the third most prevalent and widely distributed neurotransmitter in the mammalian nervous system. NAAG activates a group II metabotropic glutamate receptor (mGluR3) and is inactivated by an extracellular enzyme, glutamate carboxypeptidase II (GCPII) in vivo. Inhibitors of this enzyme are analgesic in animal models of inflammatory, neuropathic and bone cancer pain. NAAG and GCPII are present in the locus coeruleus, a center for the descending noradrenergic inhibitory pain system. In the formalin footpad model, systemic treatment with GCPII inhibitors reduces both phases of the inflammatory pain response and increases release of spinal noradrenaline. This analgesic efficacy is blocked by systemic injection of a group II mGluR antagonist, by intrathecal (spinal) injection of an alpha 2 adrenergic receptor antagonist and by microinjection of an α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor antagonist directly into the contralateral locus coeruleus. Footpad inflammation increases release of glutamate in the contralateral locus coeruleus and systemic treatment with a GCPII inhibitor blocks this increase. Direct injection of GCPII inhibitors into the contralateral or ipsilateral locus coeruleus reduces both phases of the inflammatory pain response in a dose-dependent manner and the contralateral effect also is blocked by intrathecal injection of an alpha 2 adrenergic receptor antagonist. These data support the hypothesis that the analgesic efficacy of systemically administered GCPII inhibitors is mediated, at least in part, by the contralateral locus coeruleus via group II mGluR, AMPA and alpha 2 adrenergic receptors.

Multimodal Imaging of Neurometabolic Pathology due to Traumatic Brain Injury

The impact of traumatic brain injury (TBI) involves a combination of complex biochemical processes beginning with the initial insult and lasting for days, months and even years post-trauma. These changes range from neuronal integrity losses to neurotransmitter imbalance and metabolite dysregulation, leading to the release of pro- or anti-apoptotic factors which mediate cell survival or death. Such dynamic processes affecting the brain's neurochemistry can be monitored using a variety of neuroimaging techniques, whose combined use can be particularly useful for understanding patient-specific clinical trajectories. Here, we describe how TBI changes the metabolism of essential neurochemical compounds, summarize how neuroimaging approaches facilitate the study of such alterations, and highlight promising ways in which neuroimaging can be used to investigate post-TBI changes in neurometabolism.

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.

Supplementation of Antipsychotic Treatment with the Amino Acid Sarcosine Influences Proton Magnetic Resonance Spectroscopy Parameters in Left Frontal White Matter in Patients with Schizophrenia

Dysfunction of the glutamatergic system, the main stimulating system in the brain, has a major role in pathogenesis of schizophrenia. The frontal white matter (WM) is partially composed of axons from glutamatergic pyramidal neurons and glia with glutamatergic receptors. The natural amino acid sarcosine, a component of a normal diet, inhibits the glycine type 1 transporter, increasing the glycine level. Thus, it modulates glutamatergic transmission through the glutamatergic ionotropic NMDA (N-methyl-d-aspartate) receptor, which requires glycine as a co-agonist. To evaluate the concentrations of brain metabolites (NAA, N-acetylaspartate; Glx, complex of glutamate, glutamine, and γ-aminobutyric acid (GABA); mI, myo-inositol; Cr, creatine; Cho, choline) in the left frontal WM, Proton Nuclear Magnetic Resonance (¹H-NMR) spectroscopy was used. Twenty-five patients randomly chosen from a group of fifty with stable schizophrenia (DSM-IV-TR) and dominant negative symptoms, who were receiving antipsychotic therapy, were administered 2 g of sarcosine daily for six months. The remaining 25 patients received placebo. Assignment was double blinded. ¹H-NMR spectroscopy (1.5 T) was performed twice: before and after the intervention. NAA, Glx and mI were evaluated as Cr and Cho ratios. All patients were also assessed twice with the Positive and Negative Syndrome Scale (PANSS). Results were compared between groups and in two time points in each group. The sarcosine group demonstrated a significant decrease in WM Glx/Cr and Glx/Cho ratios compared to controls after six months of therapy. In the experimental group, the final NAA/Cr ratio significantly increased and Glx/Cr ratio significantly decreased compared to baseline values. Improvement in the PANSS scores was significant only in the sarcosine group. In patients with schizophrenia, sarcosine augmentation can reverse the negative effect of glutamatergic system overstimulation, with a simultaneous beneficial increase of NAA/Cr ratio in the WM of the left frontal lobe. Our results further support the glutamatergic hypothesis of schizophrenia.

The predictive value of baseline NAA/Cr for treatment response of first-episode schizophrenia: A ¹H MRS study

The study focused on the predictive value of baseline metabolite ratios in bilateral hippocampus of first-episode schizophrenia by using proton magnetic resonance spectroscopy ((1)H MRS). (1)H MRS data were acquired from 23 hallucination and 17 non-hallucination first-episode schizophrenia patients compared with 17 healthy participants. Clinical characteristics of patients were rated using the Positive and Negative Syndrome Scale (PANSS) before and after 3-month treatment. The schizophrenia patients showed lower NAA/Cr ratio than healthy participants respectively (p=0.024; p=0.001), and non-hallucination patients had even lower NAA/Cr ratio than hallucination patients (p=0.033). After 3-month treatment, hallucination patients had greater improvement in negative symptoms than non-hallucination patients (p=0.018). The reduction of PANSS total score and negative factor score was positively correlated with the left NAA/Cr in both group patients (p<0.05). Given that the bilateral hippocampal baseline NAA/Cr had predictive value for the whole treatment response, and the left hippocampal NAA/Cr can predict the prognosis of negative symptoms during acute phase medication in first-episode schizophrenia.

Magnetic resonance spectroscopy of the ischemic brain under lithium treatment. Link to mitochondrial disorders under stroke

Recent evidence suggests that mitochondria are one of the main factors in the pathogenesis in different organs including brain. The pathogenesis after brain damage is caused not only by the change in bioenergetics, but also involves impairment of alternative functions of mitochondria, particularly those related to the control of cell death. In this study we evaluated partial metabolic pathways under the simulation of a stroke by using the occlusion of the middle cerebral artery in rats. The analysis shows that the induced switch to a non-oxidative energy metabolism (glycolysis) due to the block of tissue oxygen supply does not ensure the adequate supply of the tissue with ATP. Moreover, the well-known acidification of the ischemic tissue is not associated with the so-called traditionally and incorrectly considered "lactic acidosis" (the generation of lactate from glucose by itself does not lead to excessive generation of protons), but occurs because of the consumption of tissue ATP under its reduced resynthesis. Incubation of mitochondria isolated from normal rat brain at neutral and slightly acidic pH, mimicking the intracellular pH of normal and ischemic tissues correspondingly, revealed serious changes in mitochondrial bioenergetics, partially reflected in the magnitude of respiratory control and the basal and maximally stimulated respiration rates. Measurement of available metabolites by (1)H MR spectra of normal and ischemia-damaged brains showed a significant increase in lactate and myo-inositol and a moderate decrease in N-acetylaspartate 24h after reperfusion. Remarkably, the administration of lithium chloride in the reperfusion phase normalized the levels of metabolites. Moreover, the introduction of lithium salts (chloride or succinate) in the bloodstream, restored after ischemia, reduced both the size of the ischemia-induced brain damage and the degree of brain swelling. Besides, post-ischemic introduction of lithium salts largely restored the neurological status of the animal.

Neurochemical alterations associated with borderline personality disorder

In neuroimaging on borderline personality disorder, prior studies focused on the hippocampus and amygdala, as mentioned above. However, no study investigated whether there were neurochemical changes in the patients with borderline personality disorder. Therefore, in the present study, we aimed to investigate neurochemical change of patients diagnosed with borderline disorder and hypothesized that neurochemicals would change in the hippocampus region of these patients. Seventeen patients and the same number of healthy control subjects were analyzed by using a 1.5 Tesla GE Signa Imaging System. N-acetylaspartate (NAA), choline compounds (CHO), and creatine (CRE) values of hippocampal region were measured. The mean NAA/CRE ratio in the hippocampus region was significantly reduced in the patients with borderline personality disorder compared to that of healthy control subjects, In addition, NAA/CHO ratio of the patients with borderline personality disorder was also significantly reduced when compared to that of healthy subjects. There was no difference in the ratio of CHO/CRE. In summary, we present evidence for reduced NAA in the patients with borderline personality disorder.

N-acetylaspartate decrease in acute stage of ischemic stroke: a perspective from experimental and clinical studies

N-acetylaspartate (NAA) appears in a prominent peak in proton magnetic resonance spectroscopy ((1)H-MRS) of the brain. Exhibition by NAA of time-dependent attenuation that reflects energy metabolism during the acute stage of cerebral ischemia makes this metabolite a unique biomarker for assessing ischemic stroke. Although magnetic resonance (MR) imaging is a powerful technique for inspecting the pathological changes that occur during ischemic stroke, biomarkers that directly reflect the drastic metabolic changes associated with acute-stage ischemia are strongly warranted for appropriate therapeutic decision-making in daily clinical settings. In this review, we provide a brief overview of NAA metabolism and focus on the use of attenuation in NAA as a means for assessing the pathophysiological changes that occur during the acute stage of ischemic stroke.

Localisation of N-acetylaspartate in oligodendrocytes/myelin

The role of N-acetylaspartate in the brain is unclear. Here we used specific antibodies against N-acetylaspartate and immunocytochemistry of carbodiimide-fixed adult rodent brain to show that, besides staining of neuronal cell bodies in the grey matter, N-acetylaspartate labelling was present in oligodendrocytes/myelin in white matter tracts. Immunoelectron microscopy of the rat hippocampus showed that N-acetylaspartate was concentrated in the myelin. Also neuronal cell bodies and axons contained significant amounts of N-acetylaspartate, while synaptic elements and astrocytes were low in N-acetylaspartate. Mitochondria in axons and neuronal cell bodies contained higher levels of N-acetylaspartate compared to the cytosol, compatible with synthesis of N-acetylaspartate in mitochondria. In aspartoacylase knockout mice, in which catabolism of N-acetylaspartate is blocked, the levels of N-acetylaspartate were largely increased in oligodendrocytes/myelin. In these mice, the highest myelin concentration of N-acetylaspartate was found in the cerebellum, a region showing overt dysmyelination. In organotypic cortical slice cultures there was no evidence for N-acetylaspartate-induced myelin toxicity, supporting the notion that myelin damage is induced by the lack of N-acetylaspartate for lipid production. Our findings also implicate that N-acetylaspartate signals on magnetic resonance spectroscopy reflect not only vital neurons but also vital oligodendrocytes/myelin.

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.

In vivo glutamate measured with magnetic resonance spectroscopy: behavioral correlates in aging

Altered availability of the brain biochemical glutamate might contribute to the neural mechanisms underlying age-related changes in cognitive and motor functions. To investigate the contribution of regional glutamate levels to behavior in the aging brain, we used an in vivo magnetic resonance spectroscopy protocol optimized for glutamate detection in 3 brain regions targeted by cortical glutamatergic efferents-striatum, cerebellum, and pons. Data from 61 healthy men and women ranging in age from 20 to 86 years were used. Older age was associated with lower glutamate levels in the striatum, but not cerebellum or pons. Older age was also predictive of poorer performance on tests of visuomotor skills and balance. Low striatal glutamate levels were associated with high systolic blood pressure and worse performance on a complex visuomotor task, the Grooved Pegboard. These findings suggest that low brain glutamate levels are related to high blood pressure and that changes in brain glutamate levels might mediate the behavioral changes noted in normal aging.

Altered reward circuitry in the norepinephrine transporter knockout mouse

Synaptic levels of the monoamine neurotransmitters dopamine, serotonin, and norepinephrine are modulated by their respective plasma membrane transporters, albeit with a few exceptions. Monoamine transporters remove monoamines from the synaptic cleft and thus influence the degree and duration of signaling. Abnormal concentrations of these neuronal transmitters are implicated in a number of neurological and psychiatric disorders, including addiction, depression, and attention deficit/hyperactivity disorder. This work concentrates on the norepinephrine transporter (NET), using a battery of in vivo magnetic resonance imaging techniques and histological correlates to probe the effects of genetic deletion of the norepinephrine transporter on brain metabolism, anatomy and functional connectivity. MRS recorded in the striatum of NET knockout mice indicated a lower concentration of NAA that correlates with histological observations of subtle dysmorphisms in the striatum and internal capsule. As with DAT and SERT knockout mice, we detected minimal structural alterations in NET knockout mice by tensor-based morphometric analysis. In contrast, longitudinal imaging after stereotaxic prefrontal cortical injection of manganese, an established neuronal circuitry tracer, revealed that the reward circuit in the NET knockout mouse is biased toward anterior portions of the brain. This is similar to previous results observed for the dopamine transporter (DAT) knockout mouse, but dissimilar from work with serotonin transporter (SERT) knockout mice where Mn²⁺ tracings extended to more posterior structures than in wildtype animals. These observations correlate with behavioral studies indicating that SERT knockout mice display anxiety-like phenotypes, while NET knockouts and to a lesser extent DAT knockout mice display antidepressant-like phenotypic features. Thus, the mainly anterior activity detected with manganese-enhanced MRI in the DAT and NET knockout mice is likely indicative of more robust connectivity in the frontal portion of the reward circuit of the DAT and NET knockout mice compared to the SERT knockout mice.

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.

NAAG peptidase inhibition in the periaqueductal gray and rostral ventromedial medulla reduces flinching in the formalin model of inflammation

Background

Metabotropic glutamate receptors (mGluRs) have been identified as significant analgesic targets. Systemic treatments with inhibitors of the enzymes that inactivate the peptide transmitter N-acetylaspartylglutamate (NAAG), an mGluR3 agonist, have an analgesia-like effect in rat models of inflammatory and neuropathic pain. The goal of this study was to begin defining locations within the central pain pathway at which NAAG activation of its receptor mediates this effect.

Results

NAAG immunoreactivity was found in neurons in two brain regions that mediate nociceptive processing, the periaqueductal gray (PAG) and the rostral ventromedial medulla (RVM). Microinjection of the NAAG peptidase inhibitor ZJ43 into the PAG contralateral, but not ipsilateral, to the formalin injected footpad reduced the rapid and slow phases of the nociceptive response in a dose-dependent manner. ZJ43 injected into the RVM also reduced the rapid and slow phase of the response. The group II mGluR antagonist LY341495 blocked these effects of ZJ43 on the PAG and RVM. NAAG peptidase inhibition in the PAG and RVM did not affect the thermal withdrawal response in the hot plate test. Footpad inflammation also induced a significant increase in glutamate release in the PAG. Systemic injection of ZJ43 increased NAAG levels in the PAG and RVM and blocked the inflammation-induced increase in glutamate release in the PAG.

Conclusion

These data demonstrate a behavioral and neurochemical role for NAAG in the PAG and RVM in regulating the spinal motor response to inflammation and that NAAG peptidase inhibition has potential as an approach to treating inflammatory pain via either the ascending (PAG) and/or the descending pain pathways (PAG and RVM) that warrants further study.

Effects of N-acetylaspartylglutamate (NAAG) peptidase inhibition on release of glutamate and dopamine in prefrontal cortex and nucleus accumbens in phencyclidine model of schizophrenia

The "glutamate" theory of schizophrenia emerged from the observation that phencyclidine (PCP), an open channel antagonist of the NMDA subtype of glutamate receptor, induces schizophrenia-like behaviors in humans. PCP also induces a complex set of behaviors in animal models of this disorder. PCP also increases glutamate and dopamine release in the medial prefrontal cortex and nucleus accumbens, brain regions associated with expression of psychosis. Increased motor activation is among the PCP-induced behaviors that have been widely validated as models for the characterization of new antipsychotic drugs. The peptide transmitter N-acetylaspartylglutamate (NAAG) activates a group II metabotropic receptor, mGluR3. Polymorphisms in this receptor have been associated with schizophrenia. Inhibitors of glutamate carboxypeptidase II, an enzyme that inactivates NAAG following synaptic release, reduce several behaviors induced by PCP in animal models. This research tested the hypothesis that two structurally distinct NAAG peptidase inhibitors, ZJ43 and 2-(phosphonomethyl)pentane-1,5-dioic acid, would elevate levels of synaptically released NAAG and reduce PCP-induced increases in glutamate and dopamine levels in the medial prefrontal cortex and nucleus accumbens. NAAG-like immunoreactivity was found in neurons and presumptive synaptic endings in both regions. These peptidase inhibitors reduced the motor activation effects of PCP while elevating extracellular NAAG levels. They also blocked PCP-induced increases in glutamate but not dopamine or its metabolites. The mGluR2/3 antagonist LY341495 blocked these behavioral and neurochemical effects of the peptidase inhibitors. The data reported here provide a foundation for assessment of the neurochemical mechanism through which NAAG achieves its antipsychotic-like behavioral effects and support the conclusion NAAG peptidase inhibitors warrant further study as a novel antipsychotic therapy aimed at mGluR3.

Altered neurochemical ingredient of hippocampus in patients with bipolar depression

Background. In a number of investigations, hippocampal neurochemicals were evaluated in the patients with bipolar disorder who were on their first episode or euthymic periods. However, we did not meet any investigation in which only patients with bipolar depression were examined. As a consequence, the objective of the present study was to examine both sides of hippocampus of patients with bipolar disorder in depressive episode and healthy controls using (1)H-MRS. Methods. Thirteen patients with DSM-IV bipolar I disorder, most recent episode depressed, were recruited from the Department of Psychiatry at Firat University School of Medicine. We also studied 13 healthy comparison subjects who were without any DSM-IV Axis I disorders recruited from the hospital staff. The patients and controls underwent proton magnetic resonance spectroscopy ((1)H-MRS) of their hippocampus. NAA, CHO, and CRE values were measured. Results. No significant effect of diagnosis was observed for NAA/CRE ratio. For the NAA/CHO ratio, the ANCOVA with age, gender, and whole brain volume as covariates revealed that the patients with bipolar depression had significantly lower ratio compared to healthy control subjects for right and for left side. As for the CHO/CRE ratio, the difference was statistically significant for right side, with an effect diagnosis of F = 4.763, P = 0.038, and was very nearly significant for left side, with an effect diagnosis of F = 3.732, P = 0.064. Conclusions. We found that the patients with bipolar depression had lower NAA/CHO and higher CHO/CRE ratios compared to those of healthy control subjects. The findings of the present study also suggest that there may be a degenerative process concerning the hippocampus morphology in the patients with bipolar depression.

Valproate and neuroprotective effects for bipolar disorder

Valproate is an anticonvulsant drug but also a mood stabilizer commonly used to treat bipolar disorder. It has a structure of short-chain fatty acid and is becoming a first line treatment for bipolar disorder. The effect mechanism of the vaproate has not been completely established but it has been suggested that alterations in gene expression may be involved in chronic treatment. On the other hand, a growing body of evidence emphasizes that valproate has neuroprotective and neurotrophic actions. Neuroimaging studies that examine neurochemistry in the living brain provide further support for the hypothesis that bipolar disorder is related to changes in neuronal viability and function. In cellular view of point, it was showed that valproate protected rat cerebral cortical and cerebellar granule cells from glutamate-related excitotoxicity, and apoptotic death of the endoplasmic reticulum in C6 glioma cells and PC 12 cells. At the genetic level, growing data suggest that the long-term treatment of mood disorders may involve the regulation of signalling pathways and gene expression in critical neuronal circuits. It has been shown that lithium and valproate produce some changes in basal and stimulated DNA binding to activator protein 1 (AP-1) transcription factors, considering that strategic changes in gene expression in critical neuronal circuits may be important in the treatment of a variety of psychiatric disorders. So, a growing body of evidence establishes its neuroprotective and neurotrophic actions in bipolar disorder.

Interaction of hippocampal volume and N-acetylaspartate concentration deficits in schizophrenia: a combined MRI and 1H-MRS study

BACKGROUND

Volume deficits assessed with magnetic resonance imaging (MRI) and neurochemical dysfunctions (N-acetylaspartate, NAA) diagnosed using proton MR spectroscopy ((1)H-MRS) are reliable observations in the hippocampus of schizophrenic patients. NAA is an important cerebral amino acid in the synthesis pathways of glutamate, which has been implicated as a pathobiological core of schizophrenic symptomatology, of histological alterations and brain volume deficits in schizophrenia. However, the possible interaction between regional NAA reduction and volume deficits has been targeted only marginally in previous investigations.

METHODS

In 29 schizophrenic patients and 44 control subjects, a multimodal imaging study with (1)H-MRS and MRI volumetry of the left hippocampus was performed on a 3-Tesla scanner.

RESULTS

Compared to the control group, the hippocampus of the patients exhibited a significant volume reduction and a significant NAA concentration decrease. In schizophrenic patients, but not in healthy controls, a significant negative correlation between hippocampal NAA concentration and volume (r=-0.455, p=0.017) was observed. None of the imaging parameters was associated with clinical parameters.

CONCLUSIONS

The results argue for a coexistent neurochemical and structural deficit in the hippocampus of schizophrenic patients. The inverse relationship between the two parameters observed in patients only may reflect an interaction of neurochemistry and brain morphology as a pathobiological mechanism in schizophrenia. This observation is compatible with the important role of NAA in the synthesis of excitatory neurotransmitters and the hypothesized role of glutamate for brain morphology. The independence of the measured imaging parameters from clinical parameters is in line with the neurodevelopmental hypothesis of schizophrenia.

Methamphetamine-induced neuronal protein NAT8L is the NAA biosynthetic enzyme: implications for specialized acetyl coenzyme A metabolism in the CNS

N-acetylaspartate (NAA) is a concentrated, neuron-specific brain metabolite routinely used as a magnetic resonance spectroscopy marker for brain injury and disease. Despite decades of research, the functional roles of NAA remain unclear. Biochemical investigations over several decades have associated NAA with myelin lipid synthesis and energy metabolism. However, studies have been hampered by an inability to identify the gene for the NAA biosynthetic enzyme aspartate N-acetyltransferase (Asp-NAT). A very recent report has identified Nat8l as the gene encoding Asp-NAT and confirmed that the only child diagnosed with a lack of NAA on brain magnetic resonance spectrograms has a 19-bp deletion in this gene. Based on in vitro Nat8l expression studies the researchers concluded that many previous biochemical investigations have been technically flawed and that NAA may not be associated with brain energy or lipid metabolism. In studies done concurrently in our laboratory we have demonstrated via cloning, expression, specificity for acetylation of aspartate, responsiveness to methamphetamine treatment, molecular modeling and comparative immunolocalization that NAT8L is the NAA biosynthetic enzyme Asp-NAT. We conclude that NAA is a major storage and transport form of acetyl coenzyme A specific to the nervous system, thus linking it to both lipid synthesis and energy metabolism.

Glyceryl triacetate for Canavan disease: a low-dose trial in infants and evaluation of a higher dose for toxicity in the tremor rat model

Canavan disease (CD) is a fatal dysmyelinating genetic disorder associated with aspartoacylase deficiency, resulting in decreased brain acetate levels and reduced myelin lipid synthesis in the developing brain. Here we tested tolerability of a potent acetate precursor, glyceryl triacetate (GTA), at low doses in two infants diagnosed with CD, aged 8 and 13 months. Much higher doses of GTA were evaluated for toxicity in the tremor rat model of CD. GTA was given orally to the infants for up to 4.5 and 6 months, starting at 25 mg/kg twice daily, doubling the dose weekly until a maximum of 250 mg/kg reached. Wild-type and tremor rat pups were given GTA orally twice daily, initially at a dose of 4.2 g/kg from postnatal days 7 through 14, and at 5.8 g/kg from day 15 through 23, and thereafter in food (7.5%) and water (5%). At the end of the trial (approximately 90 to 120 days) sera and tissues from rats were analysed for changes in blood chemistry and histopathology. GTA treatment caused no detectable toxicity and the patients showed no deterioration in clinical status. In the high-dose animal studies, no significant differences in the mean blood chemistry values occurred between treated and untreated groups, and no lesions indicating toxicity were detectable in any of the tissues examined. Lack of GTA toxicity in two CD patients in low-dose trials, as well as in high-dose animal studies, suggests that higher, effective dose studies in human CD patients are warranted.

Phenotypic characterization of mice heterozygous for a null mutation of glutamate carboxypeptidase II

Glutamate is the major excitatory neurotransmitter in the mammalian central nervous system. Disturbed glutamate signaling resulting in hypofunction of N-methyl-D-aspartate receptors (NMDAR) has been implicated in the pathophysiology of schizophrenia. Glutamate Carboxypeptidase II (GCP II) hydrolyzes N-acetyl-alpha L-aspartyl-L-glutamate (NAAG) into glutamate and N-acetyl-aspartate. NAAG is a neuropeptide that is an NMDAR antagonist as well as an agonist for the metabotropic glutamate receptor-3 (mGluR3), which inhibits glutamate release. The aggregate effect of NAAG is thus to attenuate NMDAR activation. To manipulate the expression of GCP II, LoxP sites were inserted flanking exons 1 and 2, which were excised by crossing with a Cre-expressing mouse. The mice heterozygous for this deletion showed a 50% reduction in the expression level of protein and functional activity of GCP II in brain samples. Heterozygous mutant crosses did not yield any homozygous null animals at birth or as embryos (N > 200 live births and fetuses). These data are consistent with the previous report that GCP II homozygous mutant mice generated by removing exons 9 and 10 of GCP II gene were embryonically lethal and confirm our hypothesis that GCP II plays an essential role early in embryonic development. Heterozygous mice, however, developed normally to adulthood and exhibited increased locomotor activity, reduced social interaction, and a subtle cognitive deficit in working memory.

Antipsychotic drugs increase N-acetylaspartate and N-acetylaspartylglutamate in SH-SY5Y human neuroblastoma cells

N-Acetylaspartate (NAA) and N-acetylaspartylglutamate (NAAG) are related neuronal metabolites associated with the diagnosis and treatment of schizophrenia. NAA is a valuable marker of neuronal viability in magnetic resonance spectroscopy, a technique which has consistently shown NAA levels to be modestly decreased in the brains of schizophrenia patients. However, there are conflicting reports on the changes in brain NAA levels after treatment with antipsychotic drugs, which exert their therapeutic effects in part by blocking dopamine D(2) receptors. NAAG is reported to be an agonist of the metabotropic glutamate 2/3 receptor, which is linked to neurotransmitter release modulation, including glutamate release. Alterations in NAAG metabolism have been implicated in the development of schizophrenia possibly via dysregulation of glutamate neurotransmission. In the present study we have used high performance liquid chromatography to determine the effects of the antipsychotic drugs haloperidol and clozapine on NAA and NAAG levels in SH-SY5Y human neuroblastoma cells, a model system used to test the responses of dopaminergic neurons in vitro. The results indicate that the antipsychotic drugs haloperidol and clozapine increase both NAA and NAAG levels in SH-SY5Y cells in a dose and time dependant manner, providing evidence that NAA and NAAG metabolism in neurons is responsive to antipsychotic drug treatment.

N-acetylaspartate synthesis in the brain: mitochondria vs. microsomes

Several reports during the last three decades have indicated that biosynthesis of N-acetylaspartate (NAA) occurs primarily in the mitochondria. But a recent report by Lu et al. in this journal [2004; 122: 71-78] and subsequent two reports that cited those data suggested a predominant microsomal localization of the NAA biosynthetic enzyme, which is surprising in view of what is known about the biological functions of NAA. Therefore we reinvestigated this issue in rat brain homogenates using a similar fractionation procedure used by Lu et al. but without the loss of enzyme activity that they have encountered. We found that about 70% of the total Asp-NAT activity in the crude supernatant was present in the mitochondrial fraction which is about 5 times more than that in the microsomes. We found similar results in the case of the enzyme from bovine brain. In subsequent studies, we also have found that Asp-NAT activity in the bovine brain is very similar to that in the rat brain in substrate specificity and chromatographic characteristics including the high molecular weight pattern (approx. 670 kD) on size-exclusion HPLC.

Cognitive impairment and in vivo metabolites in first-episode neuroleptic-naive and chronic medicated schizophrenic patients: a proton magnetic resonance spectroscopy study

Involvement of the prefrontal cortex in schizophrenia has been implicated by neuropsychological, as well as neuropathological and imaging studies. Reductions of N-acetylaspartate (NAA), an in vivo marker of neuronal integrity, have repeatedly been detected in the frontal lobes of patients with schizophrenia by proton magnetic resonance spectroscopy (1H-MRS). In chronic medicated patients, a positive correlation between NAA levels of the prefrontal cortex and cognitive functioning has been observed, but to date, there have been no studies in first-episode neuroleptic-naive patients. In this study, single-voxel 1H-MRS was used to investigate neuronal function of the dorsolateral prefrontal cortex in 15 first-episode and 20 chronic schizophrenic patients. Outcomes were compared to 20 age-matched healthy controls to assess the relationship between prefrontal metabolism and neuropsychological performance. Patients with chronic schizophrenia had significant reductions of NAA, glutamate/glutamine, and choline levels compared to first-episode patients and healthy controls. Furthermore, creatine and phosphocreatine were significantly reduced in both patient groups compared to healthy controls. In the neuropsychological tests, chronic schizophrenic patients performed significantly poorer in the Auditory Verbal Learning Task (AVLT) compared to first-episode patients. In both patient groups, NAA levels of the left frontal lobe significantly correlated with performances in verbal learning and memory. These results corroborate data from recent structural and spectroscopic imaging studies of the frontal lobes in schizophrenia, in which cortical gray matter reductions after onset of symptoms as well as reduced levels of NAA in chronic, but not in first-episode schizophrenic patients have been reported.

Bimodal occurrence of aspartoacylase in myelin and cytosol of brain

The growing use of N-acetylaspartate as an indicator of neuronal viability has fostered interest in the biological function(s) of this unusual amino acid derivative. In considering the various physiological roles that have been proposed for this relatively abundant molecule one is obliged to take into account its unusual metabolic compartmentalization, according to which synthesis and storage occur in the neuron and hydrolytic cleavage in the oligodendrocyte. The latter reaction, catalyzed by aspartoacylase (ASPA), produces acetyl groups plus aspartate and has been proposed to occur in both soluble and membranous subfractions of white matter. Our study supports such bimodal occurrence and we now present immunoblot, proteomic, and biochemical evidence that the membrane-bound form of ASPA is intrinsic to purified myelin membranes. This was supported by a novel TLC-based method for the assay of ASPA. That observation, together with previous demonstrations of numerous lipid-synthesizing enzymes in myelin, suggests utilization of acetyl groups liberated by myelin-localized ASPA for lipid synthesis within the myelin sheath. Such synthesis might be selective and could explain the deficit of myelin lipids in animals lacking ASPA.

Endogenous N-acetylaspartylglutamate reduced NMDA receptor-dependent current neurotransmission in the CA1 area of the hippocampus

N-Acetylaspartylglutamate (NAAG) is a neuropeptide found in high concentrations in the brain. Using whole-cell recordings of CA1 pyramidal neurons in acute hippocampal slices, we found that either (i) the application of exogenous NAAG or (ii) an increase of endogenous extracellular NAAG, caused by the inhibition of its catabolic enzyme glutamate carboxypeptidase II (GCP II), resulted in a significant reduction in the amplitude of the isolated NMDA receptor (NMDAR) component of the evoked excitatory postsynaptic current (EPSC). Conversely, reduction of endogenous extracellular NAAG caused by either (i) perfusion with a soluble form of pure human GCP II or (ii) affinity purified antibodies against NAAG, enhanced the amplitude of the isolated NMDAR current. Bath application of GCP II inhibitor induced a progressive loss of spontaneous NMDAR miniatures. Furthermore, NAAG blocked the induction of long-term potentiation at Schaffer collateral axons-CA1 pyramidal neuron synapses. All together, these results suggest that NAAG acts as an endogenous modulator of NMDARs in the CA1 area of the hippocampus.

Selective vulnerability of hippocampal NAAGergic neurons in experimental temporal lobe epilepsy

The dipeptide N-acetylaspartylglutamate (NAAG) has been recently implicated in numerous neurological disorders. NAAG binds and stimulates group II metabotropic glutamate receptors producing a down-modulation of synaptic glutamate release. In the present immunohistochemical study, we compare the distribution of NAAG-containing (NAAGergic) neurons between the hippocampus of control and chronic epileptic rats obtained with the pilocarpine model of temporal lobe epilepsy. In the hippocampal formation, NAAGergic neurons comprise a subpopulation of GABAergic neurons. Examination by light microscopy revealed a significant reduction of NAAG-immunoreactive neurons in CA3 stratum oriens (35.8%) and CA1 stratum oriens (78.87%), stratum pyramidale (40%), and stratum radiatum (56.6%). Similar loss of NAAGergic neurons was observed in the subiculum characterized by 71.82% and 77.53% reduction in the stratum oriens and radiatum, respectively, when compared with controls. NAAGergic neurons in CA2 and dentate gyrus were apparently resistant to seizure-related cell loss but appeared more complex and exhibited numerous NAAG-positive puncta. Our findings indicate a selective vulnerability of NAAGergic neurons in temporal lobe epilepsy.

N-Acetylaspartate in the CNS: from neurodiagnostics to neurobiology

The brain is unique among organs in many respects, including its mechanisms of lipid synthesis and energy production. The nervous system-specific metabolite N-acetylaspartate (NAA), which is synthesized from aspartate and acetyl-coenzyme A in neurons, appears to be a key link in these distinct biochemical features of CNS metabolism. During early postnatal central nervous system (CNS) development, the expression of lipogenic enzymes in oligodendrocytes, including the NAA-degrading enzyme aspartoacylase (ASPA), is increased along with increased NAA production in neurons. NAA is transported from neurons to the cytoplasm of oligodendrocytes, where ASPA cleaves the acetate moiety for use in fatty acid and steroid synthesis. The fatty acids and steroids produced then go on to be used as building blocks for myelin lipid synthesis. Mutations in the gene for ASPA result in the fatal leukodystrophy Canavan disease, for which there is currently no effective treatment. Once postnatal myelination is completed, NAA may continue to be involved in myelin lipid turnover in adults, but it also appears to adopt other roles, including a bioenergetic role in neuronal mitochondria. NAA and ATP metabolism appear to be linked indirectly, whereby acetylation of aspartate may facilitate its removal from neuronal mitochondria, thus favoring conversion of glutamate to alpha ketoglutarate which can enter the tricarboxylic acid cycle for energy production. In its role as a mechanism for enhancing mitochondrial energy production from glutamate, NAA is in a key position to act as a magnetic resonance spectroscopy marker for neuronal health, viability and number. Evidence suggests that NAA is a direct precursor for the enzymatic synthesis of the neuron specific dipeptide N-acetylaspartylglutamate, the most concentrated neuropeptide in the human brain. Other proposed roles for NAA include neuronal osmoregulation and axon-glial signaling. We propose that NAA may also be involved in brain nitrogen balance. Further research will be required to more fully understand the biochemical functions served by NAA in CNS development and activity, and additional functions are likely to be discovered.

Regulation of N-acetylaspartate and N-acetylaspartylglutamate biosynthesis by protein kinase activators

The neuronal dipeptide N-acetylaspartylglutamate (NAAG) is thought to be synthesized enzymatically from N-acetylaspartate (NAA) and glutamate. We used radiolabeled precursors to examine NAA and NAAG biosynthesis in SH-SY5Y human neuroblastoma cells stimulated with activators of protein kinase A (dbcAMP; N6,2'-O-dibutyryl cAMP) and protein kinase C (PMA; phorbol-12-myristate-13-acetate). Differentiation over the course of several days with dbcAMP resulted in increased endogenous NAA levels and NAAG synthesis from l-[(3)H]glutamine, whereas PMA-induced differentiation reduced both. Exogenously applied NAA caused dose dependent increases in intracellular NAA levels, and NAAG biosynthesis from l-[(3)H]glutamine, suggesting precursor-product and mass-action relationships between NAA and NAAG. Incorporation of l-[(3)H]aspartate into NAA and NAAG occurred sequentially, appearing in NAA by 1 h, but not in NAAG until between 6 and 24 h. Synthesis of NAAG from l-[(3)H]aspartate was increased by dbcAMP and decreased by PMA at 24 h. The effects of PMA on l-[(3)H]aspartate incorporation into NAA were temporally biphasic. Using short incubation times (1 and 6 h), PMA increased l-[(3)H]aspartate incorporation into NAA, but with longer incubation (24 h), incorporation was significantly reduced. These results suggest that, while the neuronal production of NAA and NAAG are biochemically related, significant differences exist in the regulatory mechanisms controlling their biosynthesis.