N-acetylaspartate in the vertebrate brain: metabolism and function

Increased N-acetylaspartate in rat striatum following long-term administration of haloperidol

N-acetylaspartate (NAA) is present in high concentrations in the CNS and is found primarily in neurons. NAA is considered to be a marker of neuronal viability. Numerous magnetic resonance spectroscopy (MRS) and postmortem studies have shown reductions of NAA in different brain regions in schizophrenia. Most of these studies involved patients chronically treated with antipsychotic drugs. However, the effect of chronic antipsychotic treatment on NAA remains unclear. In the present study, we measured NAA in brain tissue taken from 43 male Long-Evans rats receiving 28.5 mg/kg haloperidol decanoate i.m. every 3 weeks for 24 weeks and from 21 controls administered with vehicle. Determination of tissue concentrations of NAA was achieved by HPLC of sections of frozen tissue from several brain regions with relevance to schizophrenia. Chronic administration of haloperidol was associated with a significant increase (+23%) in NAA in the striatum (p<0.05) when compared to controls, with no significant changes in the other regions investigated (frontal and temporal cortex, thalamus, hippocampus, amygdala, and nucleus accumbens). NAA appears to be selectively increased in the striatum of rats chronically receiving haloperidol. This increase may reflect a hyperfunction of striatal neurons and relate to the reported increase in somal size of these cells and/or the increase in synaptic density seen in this region following antipsychotic administration. The lack of effect in other regions indicates that the well-documented NAA deficits seen in chronically treated schizophrenia patients is not an effect of antipsychotic medication and may in fact be related to the disease process.

Dynamic or inert metabolism? Turnover of N-acetyl aspartate and glutathione from D-[1-13C]glucose in the rat brain in vivo

The rate of (13)C-label incorporation into both aspartyl (NAA C3) and acetyl (NAA C6) groups of N-acetyl aspartate (NAA) was simultaneously measured in the rat brain in vivo for up to 19 h of [1-(13)C]glucose infusion (n = 8). Label incorporation was detected in NAA C6 approximately 1.5 h earlier than in NAA C3 because of the delayed labeling of the precursor of NAA C3, aspartate, compared to that of NAA C6, glucose. The time courses of NAA were fitted using a mathematical model assuming synthesis of NAA in one kinetic compartment with the respective precursor pools of aspartate and acetyl coenzyme A (acetyl-CoA). The turnover rates of NAA C6 and C3 were 0.7 +/- 0.1 and 0.6 +/- 0.1 micromol/(g h) with the time constants 14 +/- 2 and 13 +/- 2 h, respectively, with an estimated pool size of 8 micromol/g. The results suggest that complete label turnover of NAA from glucose occurs in approximately 70 h. Several hours after starting the glucose infusion, label incorporation into glutathione (GSH) was also detected. The turnover rate of GSH was 0.06 +/- 0.02 micromol/(g h) with a time constant of 13 +/- 2 h. The estimated pool size of GSH was 0.8 micromol/g, comparable to the cortical glutathione concentration. We conclude that NAA and GSH are completely turned over and that the metabolism is extremely slow (< 0.05% of the glucose metabolic rate).

Cerebral oxidative stress and depression of energy metabolism correlate with severity of diffuse brain injury in rats

OBJECTIVE

The combined effect of traumatic brain injury (TBI) and secondary insult on biochemical changes of cerebral tissue is not well known. For this purpose, we studied the time-course changes of parameters reflecting ROS-mediated oxidative stress and modifications of cell energy metabolism determined in rats subjected to cerebral insult of increasing severity.

METHODS

Rats were divided into four groups: 1) sham-operated, 2) subjected to 10 minutes of hypoxia and hypotension (HH), 3) subjected to severe diffuse TBI, and 4) subjected to severe diffuse TBI + HH. Rats were killed at different times after injury, and analyses of malondialdehyde, ascorbate, high-energy phosphates, nicotinic coenzymes, oxypurines, nucleosides, and N-acetylaspartate (NAA) were made by high-performance liquid chromatography on whole-brain tissue extracts.

RESULTS

Data indicated a close relationship between degree of oxidative stress and severity of brain insult, as evidenced by the highest malondialdehyde values and lowest ascorbate levels in rats subjected to TBI + HH. Similarly, modifications of parameters related to cell energy metabolism were modulated by increasing severity of brain injury, as demonstrated by the lowest values of energy charge potential, nicotinic coenzymes, and NAA and the highest levels of oxypurines and nucleosides recorded in TBI + HH rats. Both the intensity of oxidative stress-mediated cerebral damage and perturbation of energy metabolism were minimally affected in rats subjected to HH only.

CONCLUSION

These results showed that the severity of brain insult can be graded by measuring biochemical modifications, specifically, reactive oxygen species-mediated damage, energy metabolism depression, and NAA, thereby validating the rodent model of closed-head diffuse TBI coupled with HH and proposing NAA as a marker with diagnostic relevance to monitor the metabolic state of postinjured brain.

In vivo effect of chronic hypoxia on the neurochemical profile of the developing rat hippocampus

The cognitive deficits observed in children with cyanotic congenital heart disease suggest involvement of the developing hippocampus. Chronic postnatal hypoxia present during infancy in these children may play a role in these impairments. To understand the biochemical mechanisms of hippocampal injury in chronic hypoxia, a neurochemical profile consisting of 15 metabolite concentrations and 2 metabolite ratios in the hippocampus was evaluated in a rat model of chronic postnatal hypoxia using in vivo 1H NMR spectroscopy at 9.4 T. Chronic hypoxia was induced by continuously exposing rats (n = 23) to 10% O2 from postnatal day (P) 3 to P28. Fifteen metabolites were quantified from a volume of 9-11 microl centered on the left hippocampus on P14, P21, and P28 and were compared with normoxic controls (n = 14). The developmental trajectory of neurochemicals in chronic hypoxia was similar to that seen in normoxia. However, chronic hypoxia had an effect on the concentrations of the following neurochemicals: aspartate, creatine, phosphocreatine, GABA, glutamate, glutamine, glutathione, myoinositol, N-acetylaspartate (NAA), phosphorylethanolamine, and phosphocreatine/creatine (PCr/Cr) and glutamate/glutamine (Glu/Gln) ratios (P < 0.001 each, except glutamate, P = 0.04). The increased PCr/Cr ratio is consistent with decreased brain energy consumption. Given the well-established link between excitatory neurotransmission and brain energy metabolism, we postulate that elevated glutamate, Glu/Gln ratio, and GABA indicate suppressed excitatory neurotransmission in an energy-limited environment. Decreased NAA and phosphorylethanolamine suggest reduced neuronal integrity and phospholipid metabolism. The altered hippocampal neurochemistry during its development may underlie some of the cognitive deficits present in human infants at risk of chronic hypoxia.

Sex differences in N-acetylaspartate correlates of general intelligence: an 1H-MRS study of normal human brain

Researchers have long attempted to determine brain correlates of intelligence using available neuroimaging technology including CT, MRI, PET, and fMRI. Although structural and functional imaging techniques are well suited to assess gross cortical regions associated with intelligence, the integrity and functioning of underlying white matter networks critical to coordinated cortical integration remain comparatively understudied. A relatively recent neuroimaging advance is magnetic resonance spectroscopy (MRS) which allows for interrogation of biochemical substrates of brain structure and function in vivo. In this study, we examined twenty-seven normal control subjects (17 male, 10 female) to determine whether N-acetylaspartate (NAA), a metabolite found primarily within neurons, is related to intelligence as assessed by the Wechsler Adult Intelligence Scale-III. Of the three white matter regions studied (i.e., left frontal, right frontal, left occipito-parietal), we found that a model including only left occipito-parietal white matter predicted intellectual performance [F(1,25) = 8.65, P = .007; r2 = .26], providing regional specificity to our previous findings of NAA-IQ relationships. Moreover, we found that a complex combination of left frontal and left occipito-parietal NAA strongly predicted performance in women, but not men [F(2,7) = 21.84, P < .001; adjusted r2 = .82]. Our results highlight a biochemical substrate of normal intellectual performance, mediated by sex, within white matter association fibers linking posterior to frontal brain regions.

NMDA-receptor mediated efflux of N-acetylaspartate: physiological and/or pathological importance?

N-Acetylaspartate (NAA) is a largely neuron specific dianionic amino acid present in high concentration in vertebrate brain. Many fundamental questions concerning N-acetylaspartate in brain remain unanswered. One such issue is the predominantly neuronal synthesis and largely glial catabolism which implies the existence of a regulated efflux from neurons. Here we show that transient (5 min) NMDA-receptor activation (60 microM) induces a long lasting Ca2+ -dependent efflux of N-acetylaspartate from organotypic slices of rat hippocampus. The NMDA-receptor stimulated efflux was unaffected by hyper-osmotic conditions (120 mM sucrose) and no efflux of N-acetylaspartate was evoked by high K+ -depolarization (50 mM) or kainate (300 microM). These results indicate that the efflux induced by NMDA is not related directly to either cell swelling or depolarization but is coupled to Ca2+ -influx via the NMDA-receptor. The efflux of N-acetylaspartate persisted at least 20 min after the omission of NMDA, similar to the efflux of the organic anions glutathione and phosphoethanolamine. The efflux of taurine and hypotaurine was also stimulated by NMDA but returned more quickly to basal levels. The NMDA-receptor stimulated efflux of N-acetylaspartate, glutathione, phosphoethanolamine, taurine and hypotaurine correlated with delayed nerve cell death measured 24 h after the transient NMDA-receptor stimulation. However, exogenous administration of high concentrations of N-acetylaspartate to the culture medium was non-toxic. The results suggest that Ca2+ -influx via the NMDA-receptor regulates the efflux of N-acetylaspartate from neurons which may have both physiological and pathological importance.

N-acetylaspartate: Serum marker of reperfusion in ischemic stroke

Conventional ways of monitoring reperfusion in acute ischemic stroke have several limitations. In searching for an alternative, we evaluated biochemical serum markers of stroke change in relation to reperfusion. N-acetylaspartate (NAA) is a small amino acid synthesized by neuronal mitochondria, which can be released in the extracellular space after reperfusion in animal models of brain ischemia. S100B is a well-known peripheral marker of brain damage in various neurologic diseases, including stroke. Serum samples were analyzed from 13 patients with ischemic stroke who were either treated conservatively or with recombinant tissue plasminogen activator. Blood was drawn at baseline; after 30 minutes; after 1, 2, 4, and 8 hours; and between 12 to 24 hours. Serum concentrations of NAA were analyzed using a gas chromatography-mass spectrometry method. S100B was analyzed using an automated immunoluminometric assay. Reperfusion was assessed using transcranial Doppler and clinical criteria. Reperfusion (n = 4) was associated with a transient rapid increase in serum NAA levels. Such an early rapid increase of NAA was not observed for patients with persistent occlusion at 12 to 24 hours (n = 4) and patients with no occlusion on baseline transcranial Doppler (n = 5). NAA peak levels and area under the curve values were significantly higher after reperfusion in comparison with normal transcranial Doppler findings or persistent occlusion (P = .003 and P = .05, respectively). No differences were found between these groups for S100B levels. In patients with acute ischemic stroke, serum NAA levels transiently raise after reperfusion.

Transport of N-acetylaspartate via murine sodium/dicarboxylate cotransporter NaDC3 and expression of this transporter and aspartoacylase II in ocular tissues in mouse

Canavan disease is a genetic disorder associated with optic neuropathy and the metabolism of N-acetylaspartate is defective in this disorder due to mutations in the gene coding for the enzyme aspartoacylase II. Here we show that the plasma membrane transporter NaDC3, a Na+-coupled transporter for dicarboxylates, is able to transport N-acetylaspartate, suggesting that the transporter may function in concert with aspartoacylase II in the metabolism of N-acetylaspartate. Since Canavan disease is associated with ocular complications, we investigated the expression pattern of NaDC3 and aspartoacylase II in ocular tissues in mouse by in situ hybridization. These studies show that NaDC3 mRNA is expressed in the optic nerve, most layers of the retina, retinal pigment epithelium, ciliary body, iris, and lens. Aspartoacylase II mRNA is coexpressed in most of these cell types. We conclude that transport of N-acetylaspartate into ocular tissues via NaDC3 and its subsequent hydrolysis by aspartoacylase II play an essential role in the maintenance of visual function.

Lithium and valproic acid treatment effects on brain chemistry in bipolar disorder

BACKGROUND

Prior work reported elevated gray matter (GM) lactate and Glx (glutamate + glutamine + GABA) concentrations in unmedicated patients with bipolar disorder (BP) compared with healthy controls (HC). This study examined whether lithium (Li) and valproic acid (VPA) treatment modulated these chemicals.

METHODS

A subset of previously reported BP patients were treated with Li (n = 12, 3.6 +/- 1.9 months) or VPA (n = 9, 1.4 +/- 1.7 months) and compared untreated HC subjects (n = 12, 2.9 +/- 2.4 months) using proton echo-planar spectroscopic imaging. Regression analyses (voxel gray/white composition by chemistry) were performed at each time point, and change scores computed. Metabolite relaxation and regions of interest (ROI) were also examined.

RESULTS

Across treatment, Li-treated BP subjects demonstrated GM Glx decreases (Li-HC, p =.08; Li-VPA p =.04) and GM myo-inositol increases (Li-HC p =.07; Li-VPA p =.12). Other measures were not significant. Serum Li levels were positively correlated with Glx decreases at the trend level.

CONCLUSIONS

Li treatment of BP was associated with specific GM Glx decreases and myo-inositol increases. Findings are discussed in the context of cellular mechanisms postulated to underlie Li and VPA therapeutic efficacy.

High-resolution magic angle spinning and1H magnetic resonance spectroscopy reveal significantly altered neuronal metabolite profiles in CLN1 but not in CLN3

The neuronal ceroid lipofuscinoses (NCLs) are among the most severe inherited progressive neurodegenerative disorders of children. The purpose of this study was to compare the in vivo 1.5-T 1H magnetic resonance (MR) and ex vivo 14.3-T high-resolution (HR) magic angle spinning (MAS) 1H MR brain spectra of patients with infantile (CLN1) and juvenile (CLN3) types of NCL, to obtain detailed information about the alterations in the neuronal metabolite profiles in these diseases and to test the suitability of the ex vivo HR MAS (1)H MRS technique in analysis of autopsy brain tissue. Ex vivo spectra from CLN1 autopsy brain tissue (n = 9) significantly differed from those of the control (n = 9) and CLN3 (n = 5) groups, although no differences were found between the CLN3 and the control groups. Principal component analysis of ex vivo data showed that decreased levels of N-acetylaspartate (NAA), gamma-aminobutyric acid (GABA), glutamine, and glutamate as well as increased levels of inositols characterized the CLN1 spectra. Also, the intensity ratio of lipid methylene/methyl protons was decreased in spectra of CLN1 brain tissue compared with CLN3 and control brain tissue. In concordance with the ex vivo data, the in vivo spectra of late-stage patients with CLN1 (n = 3) revealed a dramatic decrease of NAA and a proportional increase of myo-inositol and lipids compared with control subjects. Again, the spectra of patients with CLN3 (n = 13) did not differ from those of controls (n = 15). In conclusion, the ex vivo and in vivo spectroscopic findings were in good agreement within all analyzed groups and revealed significant alterations in metabolite profiles in CLN1 brain tissue but not in CLN3 compared with controls. Furthermore, HR MAS 1H MR spectra facilitated refined detection of neuronal metabolites, including GABA, and composition of lipids in the autopsy brain tissue of NCL patients.

An anti-inflammatory role for N-acetyl aspartate in stimulated human astroglial cells

Although N-acetyl-L-aspartate (NAA) has been shown to be important to myelin synthesis and osmotic regulation, the biological rationale for the high levels of NAA found in the brain remains unknown. Here, a human astroglial cell line (STTG) was treated with NAA and stimulated with ionomycin, ionomycin/PMA, or IL-1beta. PGE(2) levels in ionomycin-stimulated STTG cells decreased by 76% and > 95% at NAA concentrations of 10 and 20mM, respectively. NAA also decreased the levels of COX-2 protein and activated NF-kappaB in IL-1beta-stimulated STTG cells but had little effect on unstimulated cells. Also, NAA significantly decreased intracellular calcium levels in ionomycin/PMA-stimulated cells. NAA had no effect on total COX-2 activity or COX-2 mRNA. Acetylation of IkappaBalpha kinase, an acetylation target of aspirin, was not observed when NAA was present. These results demonstrate that NAA appears to be important in the modulation of inflammation in the human STTG astroglial cell line. The results of these findings are discussed in relation to neuronal pathologies that exhibit abnormal NAA levels within the brain.

N-Acetylaspartate synthase is bimodally expressed in microsomes and mitochondria of brain

N-Acetylaspartate (NAA) is an abundant amino acid derivative of the central nervous system that is localized primarily in neurons and has found widespread use in clinical NMR spectroscopy (MRS) as a non-invasive indicator of neuronal survival and/or viability. Its function, although still obscure, is thought to reflect its unusual metabolic compartmentalization wherein NAA synthase occurs in the neuron and aspartoacylase, the hydrolytic enzyme that removes the acetyl moiety, occurs in myelin and glia. The NAA synthase enzyme, acetyl-CoA/l-aspartate N-acetyltransferase (ANAT), was previously shown to function in mitochondria (MIT), although other subcellular fractions were apparently not examined. In this study we confirmed its presence in MIT but also found significant activity in rat brain microsomes (MIC). The reaction mixture, consisting of [(14)C]aspartate plus acetyl-CoA in Na-phosphate buffer (pH 7), gave rise to [(14)C]NAA that was separated and quantified by TLC. Reaction rates were 29.0+/-0.46 and 6.27+/-0.27 nmol/h/mg for MIC and MIT, respectively. K(m) values and pH optima were similar, and both fractions showed modest enhancement of ANAT activity with the detergents Triton CF-54 and CHAPS. Our tentative conclusion is that ANAT is bimodally targeted to MIT and a component of MIC-likely endoplasmic reticulum. ANAT activity increased in both MIC and MIT between 29 and 60 days of age but differed thereafter in that only MIT ANAT showed a decrease after 1 year.

Identification and distribution of aspartoacylase in the postnatal rat brain

Aspartoacylase metabolizes N-acetylaspartic acid to produce L-aspartate and acetate. An aspartoacylase deficiency in humans is responsible for Canavan disease, a lethal autosomal recessive leukodystrophy. The role of aspartoacylase in the mammalian brain is unclear. Here we have generated and characterized a highly specific polyclonal antibody against aspartoacylase which recognizes a 37 kDa monomer and a dimer in normal but not in aspartoacylase-deficient rat tissue. Aspartoacylase protein expression sharply increases at P14, peaks at P28 and plateaus thereafter. Biochemical analysis reveals immunoreactivity in cytosolic but not in membrane fractions. Histologically, most abundant expression was observed in white matter tracts and thalamus. On the cellular level, aspartoacylase immunoreactivity is restricted to oligodendrocyte somata in both white and gray matter.

Perinatal Iron Deficiency Alters the Neurochemical Profile of the Developing Rat Hippocampus

Cognitive deficits in human infants at risk for gestationally acquired perinatal iron deficiency suggest involvement of the developing hippocampus. To understand the plausible biological explanations for hippocampal injury in perinatal iron deficiency, a neurochemical profile of 16 metabolites in the iron-deficient rat hippocampus was evaluated longitudinally by 1H NMR spectroscopy at 9.4 T. Metabolites were quantified from an 11-24 microL volume centered in the hippocampus in 18 iron-deficient and 16 iron-sufficient rats on postnatal day (PD) 7, PD10, PD14, PD21 and PD28. Perinatal iron deficiency was induced by feeding the pregnant dam an iron-deficient diet from gestational d 3 to PD7. The brain iron concentration of the iron-deficient group was 60% lower on PD7 and 19% lower on PD28 (P < 0.001 each). The concentration of 12 of the 16 measured metabolites changed over time between PD7 and PD28 in both groups (P < 0.001 each). Compared with the iron-sufficient group, phosphocreatine, glutamate, N-acetylaspartate, aspartate, gamma-aminobutyric acid, phosphorylethanolamine and taurine concentrations, and the phosphocreatine/creatine ratio were elevated in the iron-deficient group (P < 0.02 each). These neurochemical alterations suggest persistent changes in resting energy status, neurotransmission and myelination in perinatal iron deficiency. An altered neurochemical profile of the developing hippocampus may underlie some of the cognitive deficits observed in human infants with perinatal iron deficiency.

Brain damage results in down-regulation of N-acetylaspartate as a neuronal osmolyte

N-acetyl-L-aspartate (NAA) is present in the vertebrate brain, where its concentration is one of the highest of all free amino acids. Although NAA is synthesized and stored primarily in neurons, it is not hydrolyzed in these cells. However, after its regulated release into extracellular fluid, neuronal NAA is hydrolyzed by amidohydrolase II that is present in oligodendrocytes. About 30% of neurons do not contain appreciable amounts of NAA, but its prominence in 1H nuclear magnetic resonance spectroscopic (MRS) studies has led to its wide use as a neuronal marker in diagnostic human medicine as both an indicator of brain pathology, and of disease progression in a variety of central nervous system (CNS) diseases. Loss of NAA has been interpreted as indicating either loss of neurons, or loss of neuron viability. In this investigation, the upregulation of NAA in early stages of construction of the CNS, and its downregulation in experimentally induced damage models of the CNS is reported. The results of this study indicate that the buildup of NAA is not required for viability of neurons in monocellular cultures, and that NAA is lost from multicellular cultured brain slice explants that contain viable neurons. Thus, loss of NAA does not necessarily indicate either loss of neurons or their function. The NAA system, when present in the brain, appears to reflect a high degree of cellular integration, and therefore may be a unique metabolic construct of the intact vertebrate brain.