N-Acetylaspartate as a reservoir for glutamate

Medical Hypotheses (2006) 67, 506–512

http://intl.elsevierhealth.com/journals/mehy

N-Acetylaspartate as a reservoir for glutamate Joseph F. Clark a,*, Amos Doepke a, Jessica A. Filosa b, Robert L. Wardle c, Aigang Lu a, Timothy J. Meeker a, Gail J. Pyne-Geithman a a b c

Department of Neurology, University of Cincinnati, Cincinnati, OH 45267-0536, United States Department of Psychiatry, University of Cincinnati, United States Department of Molecular and Cellular Physiology, University of Cincinnati, United States

Received 21 February 2006; accepted 28 February 2006

Summary N-acetylaspartate (NAA) is an intermediary metabolite that is found in relatively high concentrations in the human brain. More specifically, NAA is so concentrated in the neurons that it generates one of the most visible peaks in nuclear magnetic resonance (NMR) spectra, thus allowing NAA to serve as ‘‘a neuronal marker’’. However, to date there is no generally accepted physiological (primary) role for NAA. Another molecule that is found at similar concentrations in the brain is glutamate. Glutamate is an amino acid and neurotransmitter with numerous functions in the brain. We propose that NAA, a six-carbon amino acid derivative, is converted to glutamate (five carbons) in an energetically favorable set of reactions. This set of reactions starts when aspartoacylase converts the six carbons of NAA to aspartate and acetate, which are subsequently converted to oxaloacetate and acetyl CoA, respectively. Aspartylacylase is found in astrocytes and oligodendrocytes. In the mitochondria, oxaloacetate and acetyl CoA are combined to form citrate. Requiring two steps, the citrate is oxidized in the Kreb’s cycle to a-ketoglutarate, producing NADH. Finally, a-ketoglutarate is readily converted to glutamate by transaminating the a-keto to an amine. The resulting glutamate can be used by multiple cells types to provide optimal brain functional and structural needs. Thus, the abundant NAA in neuronal tissue can serve as a large reservoir for replenishing glutamate in times of rapid or dynamic signaling demands and stress. This is beneficial in that proper levels of glutamate serve critical functions for neurons, astrocytes, and oligodendrocytes including their survival. In conclusion, we hypothesize that NAA conversion to glutamate is a logical and favorable use of this highly concentrated metabolite. It is important for normal brain function because of the brain’s relatively unique metabolic demands and metabolite fluxes. Knowing that NAA is converted to glutamate will be important for better understanding myriad neurodegenerative diseases such as Canavan’s Disease and Multiple Sclerosis, to name a few. Future studies to demonstrate the chemical, metabolic and pathological links between NAA and glutamate will support this hypothesis. c 2006 Elsevier Ltd. All rights reserved.



Introduction * Corresponding author. Tel.: +1 513 558 7085; fax: +1 513 558 7009. E-mail address: [email protected] (J.F. Clark).



The amino acid derivative N-acetylaspartate (NAA) is one of the most abundant molecules in the

0306-9877/$ - see front matter c 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.mehy.2006.02.047

N-Acetylaspartate and glutamate human brain, found in concentrations up to 10 mM [1,2]. NAA accounts for approximately 1% of the brain’s dry weight and nearly 7% of the osmolarity in neurons (20 mM NAA) [1,2]. As such, it is one of the most concentrated molecules in the human brain. There are several hypotheses concerning the role of N-acetylaspartate (NAA), but there is little agreement as to what its primary function is in the brain. NAA is an amino acid derivative found in the human brain in concentrations up to 10 mM [1,2]. Because it is largely localized in neurons, its neuronal concentration is approximately 20 mM [1,2], therefore, it is often used during proton magnetic resonance (1H NMR) experiments as a neuronal marker. NAA is synthesized in neuronal mitochondria [3,4], while immunostaining suggests NAA degradation (via aspartoacylase) may be a membrane associated process in the oligodendrocytes [5,6]. Astrocytes have also been shown to express aspartoacylase activity [7]. Spectra from 1H NMR spectroscopy studies observe NAA as one of the most visible peaks [8]. There are numerous reports using NAA as a neuronal marker and using it to assess diseases of altered NAA metabolism [1,2,9–14]. NAA tends to fall with stroke or acquired brain injury as well as diseases such as Canavan’s disease [7,15–19]. Despite such intense study, there is relatively little hard evidence concerning what it does in the brain. What is needed is a better understanding of the role of NAA in the human brain. In this paper, we propose that NAA is a reservoir for protecting the concentration of glutamate, a neurotransmitter, when glutamate flux is rapid. Glutamate is an amino acid found in high concentrations in the brain and there have been proposed mechanisms or pathways that link glutamate oxidation to NAA production [20]. Glutamate is a multifaceted amino acid involved in ammonia metabolism, signal transduction, synthesis of other neurotransmitters, protein synthesis, synaptic transmission and an intermediate for metabolism [21–23]. It is also thought that glutamate concentration is controlled and maintained at low levels by having the ability to rapidly produce it [24]. It is therefore, quite logical that it would have a molecular reservoir of the magnitude available via NAA.

Significance Because of the lack of agreement on the role of one of the most concentrated molecules in the human brain, this hypothesis provides an important new avenue in the understanding of brain metabolism and biochemistry. This is because the normal phys-

507 iological role for NAA is unclear and the mechanism for its observed fall during pathology is unknown as well [1,2,7,9–19]. If we discover that the flux of NAA to glutamate is enhancing glutamate-induced neuro-excito-toxicity, we could use this knowledge to develop medicaments to prevent this toxicity by blocking NAA transport into the oligodendrocytes and possibly astrocytes. Further, this hypothesis could have a substantial impact on measurements of NAA and glutamate or other metabolites using NMR where data are often reported in ratios.

The hypothesis In Fig. 1, we present the structural scheme whereby NAA is converted in six steps to glutamate. Importantly, every reaction listed in these six steps is already known to occur and as outlined in Fig. 2, the sum of these reactions is energetically favorable. Step 1. NAA fi aspartate + acetate Starting from NAA (in the oligodendrocytes and/or astrocytes), it is hydrolyzed to aspartate and acetate via aspartylacylase (EC 3.5.1.15) (Fig. 1). The two products, acetate and aspartate are important metabolites and intermediates that can be used in multiple pathways. Step 2. Acetate + CoA + ATP fi acetyl CoA + AMP + PPi In our scheme the acetate is converted to acetyl CoA via acetyl CoA synthetase (EC 6.2.1.1). Aspartate can be transaminated via aspartate aminotransferase (EC 2.6.1.1) using the following reaction;Aspartate + aketoglutarate fi glutamate + oxaloacetate. It must be noted that the glutamate produced here is not the end point of our thesis, but rather an acceptor for the amino group removed from aspartate. This reaction also uses pyrophosphate (PPi) as a cofactor. Step 3. Oxaloacetate + acetyl CoA fi citrate Note, that oxaloacetate and acetyl CoA are important constituents of the Kreb’s cycle. These two compounds can be used to form citrate in the mitochondrial matrix via citrate synthase (EC 2.3.3.1). Step 4. Citrate conversion to a-ketoglutarate Step 4 is actually two distinct reactions of the Kreb’s cycle. Citrate is converted to isocitrate via aconitase (EC 4.2.1.3). Iso-

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Oligodendrocyte and/or Astrocyte

O

Acetyl CoA OH

-

O

OH -

O

O OH

NH2

8

O

O

NH

1

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-

-

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aspartate

NH2

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aspartate

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Neuron α-ketoglutarate

ATP

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Transamination

7 glutamate

2b AMP + PPi

glutamate NH2

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glutamate

3 OO

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α-ketoglutarate

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Fatty acid synthesis

glutamine

Mitochondrial Matrix 4b -O

O

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oxaloacetate

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GABA

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oxaloacetate

Acetyl CoA

O -

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NADH + CO2

NAD

O

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-

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OH

O -

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isocitrate

citrate

Kreb’s Cycle/ Anaplerosis

Figure 1 In this figure we see the catalytic cycle that will convert NAA to glutamate in steps 1–5. This set of reactions is energetically favorable with the evolution of NADH. In steps 6–8 NAA can be regenerated. Each one of the enzymes represented in steps 1–8 exists and function in the human brain, albeit in different cells. Thus this is an intercellular metabolite shuttle system that will provide the brain with a readily replenish-able supply of substrate for rapid production of glutamate. GABA; c-amino butyric acid, CoA; coenzyme A.

Energy costs for glutamate production from NAA NAA + KG + CoA + AT P + NAD

NAA + AT P + NAD

KG + Glut + CO2 + CoA + AMP +PPi + NADH

Glut + CO2 + AMP +PPi + NADH

Figure 2 In this figure we see the relative costs for the production of glutamate from NAA. Of importance is that this conversion consumes one ATP to AMP plus PPi and produces one NADH. Thus this is an energetically favorable production. While the favorable energy costs are not essential to the hypothesis it does make the conversion of NAA to glutamate an advantageous strategy for maintaining NAA during times of stress and/or energy demand. Glut, glutamate; PPi, pyrophosphate; CoA, Coenzyme A; aKG, a-ketoglutarate.

citrate undergoes an oxidative decarboxylation via isocitrate dehydrogenase (EC 1.1.1.42) to form a-ketoglutarate. Isocitrate + NAD fi CO2 + NADH + a-ketoglutarate Step 5. a-Ketoglutarate conversion to glutamatea-Ketoglutarate is an integral part of energy and signaling metabolism as it can continue through the Kreb’s cycle,

be used for amino acid synthesis and the synthesis of signaling molecules such as glutamate and c-aminobutyric acid (GABA). In Step 5, we have a-ketoglutarate being converted via transamination to glutamate. This concludes the enzymatic steps for our hypothesis. We have made glutamate from NAA with the net cost of this flux demonstrated in Fig. 2.

N-Acetylaspartate and glutamate Based on the balanced energy equation in Fig. 2 we see that the total energy cost for the degradation of NAA to glutamate is ATP to AMP. However, if one assumes that the NADH produced via the oxidative decarboxylation of isocitrate produces 2.5 ATPs we have a NET energy gain of 0.5 ATPs per glutamate produced from NAA.

Assumptions Our hypothesis as stated above requires us to make several assumptions. First, our hypothesis suggests that the conversion of NAA to glutamate is energetically favorable. Enzymatically for reactions 1–6 this is the case, however, we have not included the costs of transport between cells or compartments. This is because the current dogma predicts that these metabolites are traveling down their concentration gradients via facilitated diffusion [25–27]. Thus, the energy costs of intercellular transport are assumed to be minimal. There is no convincing evidence of NAA active transport against an NAA gradient [28,29]. Second, in Fig. 1, we have presented a ‘cycle’ to characterize the NAA to glutamate and glutamate to NAA conversion. Dynamic metabolism with NAA has been suggested with NAA turning over every 16.7 h in humans [30]. This is thought to occur by its efflux into the extracellular fluid, associated with cycling between an anabolic synthesis of NAA in neurons and a catabolic deacetylation of NAA in oligodendrocytes [31]. Reports of such cycling of NAA (not necessarily via glutamate/a-ketoglutarate) support our concept of NAA cycling as part of the brain’s energy metabolism, including ‘looping’ through NAA. For synthesis of glutamate after NAA has been degraded in the oligodendrocytes, we are not restricting our thesis to the oligodendrocytes as exclusively using the resultant aspartate and acetate for conversion to glutamate. These metabolites may be used in the oligodendrocytes, or transported to other cells for synthesis to glutamate as well as conversion back to NAA. Our main focus in this paper is to address NAA to glutamate however, we would be remiss to assume that glutamate is a dead end. Therefore, we speculate that glutamate can be converted back to NAA (Fig. 1). Others have speculated on the re-cycling of NAA and there are reports that 5% of NAA can be re-assembled after cleavage by aspartylacylase [17,32]. Third, we have assumed that this cycle occurs in multiple tissues, multiple cells and multiple compartments. While diffusion distances may appear to be problematic, examples of similar cycles

509 (the Cori Cycle, the Lactate cycle) suggest that such metabolic buffering and cycling systems can operate in this way [33,34].

Discussion and evaluation of the hypothesis The lack of solid biochemical information concerning the role of NAA in the brain is stunning. This molecule is arguably the second or third most concentrated molecule in the human brain behind water. The roles proposed for NAA are not widely agreed upon and this lack of understanding is making it difficult to interpret studies where NAA changes are measured [35]. While our goal in this paper is to propose a central role for NAA in glutamate metabolism we wish to briefly address some key hypotheses and why we believe our proposed role better fits the current state of knowledge concerning NAA.

NAA and NAAG formation NAA has been suggested to be an intermediate metabolite in the formation of NAA-glutamate (NAAG) [36], a known functional neurotransmitter [37]. While NAA may be a step in the formation of NAAG, the relatively large concentration of NAA in the brain seems superfluous for the relatively small concentration and flux of NAAG. NAAG is about 10% of the concentration of NAA [38] and NAA’s role as a neurotransmitter is unclear [25,39,40]. Notwithstanding, NAAG’s role as a signaling molecule is not being challenged by this hypothesis. What remains unclear, is NAA’s role to ensure a supply of glutamate.

NAA as an osmolite NAA has also been proposed to be an osmolite to modulate water balance in the brain [17,31]. Neuronal metabolized water produced from oxidative metabolism is transported against somewhat of a water gradient into the extracellular fluid [41]. However, known osmolites such as potassium and taurine are already present in the brain and are known to be utilized as such [42]. Further, NAA’s relative localization in certain cells in the brain makes it difficult to reconcile its cell specific osmoregulation at the apparent expense of other cells. That is, osmoregulation for the neurons at the expense of transport of water into the oligodendrocytes. Thus, osmoregulation as a primary role for NAA is doubtful.

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NAA and anapleurosis

NAA and myelination

It noteworthy that the NAA glutamate cycle and Kreb’s cycle both have common intermediates. NAA is a structural analogue of citrate and can be readily re-arranged to citrate (after transamination of aspartate) and thereby may act as an anapleurotic intermediate (Fig. 1). In tissues such as muscle, the mitochondria can use a wealth of amino acids for supplying the Kreb’s cycle and the Kreb’s cycle intermediates can readily be used as a resource for synthesis of numerous metabolites. NAA’s anapleurotic role may be especially important for the dynamic energy demand of the neurons and oligodendrocytes. The brain is in a unique metabolic quandary in that it is somewhat restricted, with respect to availability and use of anapleurotic substrates. If the brain’s mitochondria were required to use Kreb’s cycle intermediates for glutamate and neurotransmitter synthesis, oxidative metabolism would be impaired during dynamic energy demands, such as multiple action potentials requiring neurotransmitter release. The brain needs a reservoir of precursors for making glutamate [24]. Thus, NAA can be used to protect Kreb’s cycle intermediates, including replacing them when needed, and buffering glutamate concentrations when needed.

NAA is also thought to be important in myelination by supplying fatty acid synthesis with acetyl CoA. This is somewhat supported by several reports, discussed below. First, at the cellular level, the predominate expression of aspartoacylase activity is seen in the oligodendrocyte-type-2 astrocyte progenitor cells that gives rise to the myelinating cells of the CNS [46]. However, developmental and regional distribution of aspartoacylase in rat brain is not consistent with myelination during development. Also, aspartoacylase activity in whole tissue is about 25 times greater in white matter compared to gray matter [47], with the largest developmental increases seen in regions of greatest myelination. High levels of NAA and aspartoacylase activity are found in the oligodendrocyte-type-2 astrocyte progenitor lineage of oligodendrocytes [7]. Where the role of NAA in myelin synthesis becomes questionable is that NAA metabolism appears to undergo its greatest development after myelination is complete [48]. Further, NAA levels fall when demyelination may not be occurring [19]. For example, NAA turns over at a rate of greater than once per day, while the synthesis rate for myelin in a healthy adult would not demand such a flux rate. Therefore, we believe that a central role for the NAA system in the brain cannot be accounted for by myelin synthesis alone. Finally, Canavan’s disease, a defect in NAA metabolism, is associated with demyelination [15]. The deficiency in aspartoacylase does not appear to hinder the initial myelination in the developing brain. Also, during cerebral ischemia, in which there is swelling of the brain cells, there is a 5-fold increase in extracellular NAA levels but a relative fall in 1H NMR visible NAA [49,50]. NAA falls to about 40% immediately after stroke and is no longer visible days later [19]. This rapid fall is too quick for the aspartate to be adequately used for myelin synthesis.

NAA transport It is fairly well established that NAA can be taken into cells. Intracerebrally injected radiolabelled NAA is rapidly incorporated into brain lipid fractions with increased efficiency with age [43,44]. The highest proportion of labeled lipids was found in the myelin and mitochondrial fractions. Uptake studies with labeled NAA suggested that it is taken up almost exclusively by astrocytes [45], with very little NAA uptake by neurons [16]. This is consistent with the observation of Baslow et al. [5] who demonstrated that the expression of aspartoacylase activity in cultured rat cells is limited to oligodendrocytes. It has been shown that the uptake of NAA by glia is an electrogenic process, mediated by sodium/dicarboxylate transporter NaDC3 [29] and that, at least in ocular tissues, NaDC3 and aspartoacylase are coexpressed [28]. This suggests the role of NaDC3 is partially to transport NAA from the extracellular space to the glia in order for NAA to be broken down by aspartoacylase. Thus intercellular transport of NAA for shuttling of metabolic and neurotransmitter substrates is plausible.

Conclusion In summary we have proposed that NAA can be converted to glutamate in the brain and that this glutamate can be utilized as part of the signaling and metabolic processes of the brain. This pathway is energetically favorable and in part is consistent with the relatively high concentrations of NAA in the brain. The cycle of NAA to glutamate (via aketoglutarate) and back to NAA is a multi step, multi cell, and multi compartment cycle that is

N-Acetylaspartate and glutamate likely to be tightly controlled but capable of responding quickly and dynamically when necessary. We firmly believe that a better understanding of brain metabolism, by understanding NAA metabolism, will be an important break through in brain physiology, biochemistry and pathology and will have clinical ramifications concerning in vivo studies utilizing 1H MRS measurements of NAA.

Acknowledgements Funding support from the NIH via Grants NS050569, NS049172, NS042308 to J.F.C., and from The American Heart Association to J.A.F. and R.L.W., is gratefully acknowledged.

References [1] Miller BL. A review of chemical issues in 1H NMR spectroscopy: N-acetyl-L-aspartate, creatine and choline. NMR Biomed 1991;4(2):47–52. [2] Vielhaber S, Kudin AP, Kudina TA, Stiller D, Scheich H, Schoenfeld A, et al. Hippocampal N-acetyl aspartate levels do not mirror neuronal cell densities in creatine-supplemented epileptic rats. Eur J Neurosci 2003;18(8): 2292–300. [3] Reichelt KL, Kvamme E. Acetylated and peptide bound glutamate and sapartate in brain. J Neurochem 1967; 14(10):987–95. [4] Truckenmiller ME, Namboodiri MA, Brownstein MJ, Neale JH. N-Acetylation of L-aspartate in the nervous system: differential distribution of a specific enzyme. J Neurochem 1985;45(5):1658–62. [5] Baslow MH, Suckow RF, Sapirstein V, Hungund BL. Expression of aspartoacylase activity in cultured rat macroglial cells is limited to oligodendrocytes. J Mol Neurosci 1999; 13(1–2):47–53. [6] Kirmani BF, Jacobowitz DM, Kallarakal AT, Namboodiri MA. Aspartoacylase is restricted primarily to myelin synthesizing cells in the CNS: therapeutic implications for Canavan disease. Brain Res Mol Brain Res 2002;107(2):176–82. [7] Bhakoo KK, Pearce D. In vitro expression of N-acetyl aspartate by oligodendrocytes: implications for proton magnetic resonance spectroscopy signal in vivo. J Neurochem 2000;74(1):254–62. [8] Mehta V, Namboodiri MA. N-acetylaspartate as an acetyl source in the nervous system. Brain Res Mol Brain Res 1995;31(1–2):151–7. [9] Barkhof F, van Walderveen M. Characterization of tissue damage in multiple sclerosis by nuclear magnetic resonance. Philos Trans R Soc Lond B Biol Sci 1999;354(1390): 1675–86. [10] Jenkins BG, Klivenyi P, Kustermann E, Andreassen OA, Ferrante RJ, Rosen BR, et al. Nonlinear decrease over time in N-acetyl aspartate levels in the absence of neuronal loss and increases in glutamine and glucose in transgenic Huntington’s disease mice. J Neurochem 2000;74(5): 2108–19. [11] Friedman SD, Shaw DW, Artru AA, Richards TL, Gardner J, Dawson G, et al. Regional brain chemical alterations in young children with autism spectrum disorder. Neurology 2003;60(1):100–7.

511 [12] Usenius T, Usenius JP, Tenhunen M, Vainio P, Johansson R, Soimakallio S, et al. Radiation-induced changes in human brain metabolites as studied by 1H nuclear magnetic resonance spectroscopy in vivo. Int J Radiat Oncol Biol Phys 1995;33(3):719–24. [13] Mullins PG, Rowland L, Bustillo J, Bedrick EJ, Lauriello J, Brooks WM. Reproducibility of 1H-MRS measurements in schizophrenic patients. Magn Reson Med 2003;50(4):704–7. [14] Brooks WM, Friedman SD, Stidley CA. Reproducibility of 1HMRS in vivo. Magn Reson Med 1999;41(1):193–7. [15] Madhavarao CN, Arun P, Moffett JR, Szucs S, Surendran S, Matalon R, et al. Defective N-acetylaspartate catabolism reduces brain acetate levels and myelin lipid synthesis in Canavan’s disease. Proc Natl Acad Sci USA 2005;102(14): 5221–6. Epub 2005 Mar 22. [16] Baslow MH, Resnik TR. Canavan disease. Analysis of the nature of the metabolic lesions responsible for development of the observed clinical symptoms. J Mol Neurosci 1997;9(2):109–25. [17] Baslow MH. Brain N-acetylaspartate as a molecular water pump and its role in the etiology of Canavan disease: a mechanistic explanation. J Mol Neurosci 2003;21(3):185–90. [18] Wardlaw JM, Marshall I, Wild J, Dennis MS, Cannon J, Lewis SC. Studies of acute ischemic stroke with proton magnetic resonance spectroscopy: relation between time from onset, neurological deficit, metabolite abnormalities in the infarct, blood flow, and clinical outcome. Stroke 1998;29(8):1618–24. [19] Liu YJ, Chen CY, Chung HW, Huang IJ, Lee CS, Chin SC, et al. Neuronal damage after ischemic injury in the middle cerebral arterial territory: deep watershed versus territorial infarction at MR perfusion and spectroscopic imaging. Radiology 2003;229(2):366–74. Epub 2003 Sep 25. [20] Madhavarao CN, Chinopoulos C, Chandrasekaran K, Namboodiri MA. Characterization of the N-acetylaspartate biosynthetic enzyme from rat brain. J Neurochem 2003;86(4):824–35. [21] Sibson NR, Mason GF, Shen J, Cline GW, Herskovits AZ, Wall JE, et al. In vivo (13)C NMR measurement of neurotransmitter glutamate cycling, anaplerosis and TCA cycle flux in rat brain during. J Neurochem 2001;76(4):975–89. [22] Kreibich TA, Chalasani SH, Raper JA, Chatziioannou A, Palaiologos G, Kolisis FN. The neurotransmitter glutamate reduces axonal responsiveness to multiple repellents through the activation of metabotropic glutamate receptor 1. J Neurosci 2004;24(32):7085–95. [23] Seal RP, Edwards RH. Functional implications of neurotransmitter co-release: glutamate and GABA share the load. Curr Opin Pharmacol 2005;13:13. [24] Yudkoff M, Daikhin Y, Nissim I, Horyn O, Luhovyy B, Lazarow A. Brain amino acid requirements and toxicity: the example of leucine. J Nutr 2005;135(Suppl. 6):1531S–8S. [25] Coyle JT. The nagging question of the function of N-acetylaspartylglutamate. Neurobiol Dis 1997;4(3–4):231–8. [26] Miller SL, Daikhin Y, Yudkoff M. Metabolism of N-acetyl-Laspartate in rat brain. Neurochem Res 1996;21(5):615–8. [27] Chakraborty G, Mekala P, Yahya D, Wu G, Ledeen RW. Intraneuronal N-acetylaspartate supplies acetyl groups for myelin lipid synthesis: evidence for myelin-associated aspartoacylase. J Neurochem 2001;78(4):736–45. [28] George RL, Huang W, Naggar HA, Smith SB, Ganapathy V, Wang H, et al. Transport of N-acetylaspartate via murine sodium/dicarboxylate cotransporter NaDC3 and expression of this transporter and aspartoacylase II in ocular tissues in mouse. Biochim Biophys Acta 2004;1690(1):63–9. [29] Huang W, Wang H, Kekuda R, Fei YJ, Friedrich A, Wang J, et al. Transport of N-acetylaspartate by the Na(+)-dependent

512

[30]

[31]

[32]

[33]

[34]

[35]

[36]

[37]

[38]

[39]

Clark et al. high-affinity dicarboxylate transporter NaDC3 and its relevance to the expression of the transporter in the brain. J Pharmacol Exp Ther 2000;295(1):392–403. Baslow MH. Evidence supporting a role for N-acetyl-Laspartate as a molecular water pump in myelinated neurons in the central nervous system. An analytical review. Neurochem Int 2002;40(4):295–300. Baslow MH, Dyakin VV, Nowak KL, Hungund BL, Guilfoyle DN. 2-PMPA, a NAAG peptidase inhibitor, attenuates magnetic resonance BOLD signals in brain of anesthetized mice: evidence of a link between neuron NAAG release and hyperemia. J Mol Neurosci 2005;26(1):1–15. Karelson G, Ziegler A, Kunnecke B, Seelig J. Feeding versus infusion: a novel approach to study the NAA metabolism in rat brain. NMR Biomed 2003;16(6–7):413–23. Gladden LB. Lactate metabolism: a new paradigm for the third millennium. J Physiol 2004;558(Pt 1):5–30. Epub 2004 May 6. Aubert A, Costalat R, Magistretti PJ, Pellerin L. Brain lactate kinetics: Modeling evidence for neuronal lactate uptake upon activation. Proc Natl Acad Sci USA 2005; 102(45):16448–53. Epub 2005 Oct 31. Jessen F, Traeber F, Freymann N, Maier W, Schild HH, Heun R, et al. A comparative study of the different N-acetylaspartate measures of the medial temporal lobe in Alzheimer’s disease. Dement Geriatr Cogn Disord 2005; 20(2–3):178–83. Epub 2005 Jul 15. Baslow MH. Functions of N-acetyl-L-aspartate and N-acetylL-aspartylglutamate in the vertebrate brain: role in glial cell-specific signaling. J Neurochem 2000;75(2):453–9. Neale JH, Bzdega T, Wroblewska B. N-Acetylaspartylglutamate: the most abundant peptide neurotransmitter in the mammalian central nervous system. J Neurochem 2000; 75(2):443–52. Passani LA, Vonsattel JP, Carter RE, Coyle JT. N-acetylaspartylglutamate, N-acetylaspartate, and N-acetylated alpha-linked acidic dipeptidase in human brain and their alterations in Huntington and Alzheimer’s diseases. Mol Chem Neuropathol 1997;31(2):97–118. Yan HD, Ishihara K, Serikawa T, Sasa M. Activation by N-acetyl-L-aspartate of acutely dissociated hippocampal neurons in rats via metabotropic glutamate receptors. Epilepsia 2003;44(9):1153–9.

[40] Urenjak J, Williams SR, Gadian DG, Noble M. Specific expression of N-acetylaspartate in neurons, oligodendrocyte-type-2 astrocyte progenitors, and immature oligodendrocytes in vitro. J Neurochem 1992;59(1):55–61. [41] Baslow MH, Guilfoyle DN. Effect of N-acetylaspartic acid on the diffusion coefficient of water: a proton magnetic resonance phantom method for measurement of osmolyte-obligated water. Anal Biochem 2002;311(2):133–8. [42] Hussy N, Bres V, Rochette M, Duvoid A, Alonso G, Dayanithi G, et al. Osmoregulation of vasopressin secretion via activation of neurohypophysial nerve terminals glycine receptors by glial taurine. J Neurosci 2001; 21(18):7110–6. [43] Burri R, Steffen C, Herschkowitz N. N-acetyl-L-aspartate is a major source of acetyl groups for lipid synthesis during rat brain development. Dev Neurosci 1991;13(6):403–11. [44] D’Adamo Jr AF, Yatsu FM. Acetate metabolism in the nervous system. N-acetyl-L-aspartic acid and the biosynthesis of brain lipids. J Neurochem 1966;13(10): 961–5. [45] Sager TN, Thomsen C, Valsborg JS, Laursen H, Hansen AJ. Astroglia contain a specific transport mechanism for Nacetyl-L-aspartate. J Neurochem 1999;73(2):807–11. [46] Bhakoo KK, Craig TJ, Styles P. Developmental and regional distribution of aspartoacylase in rat brain tissue. J Neurochem 2001;79(1):211–20. [47] Kaul R, Casanova J, Johnson AB, Tang P, Matalon R. Purification, characterization, and localization of aspartoacylase from bovine brain. J Neurochem 1991;56(1): 129–35. [48] Mathew R, Arun P, Madhavarao CN, Moffett JR, Namboodiri MA. Progress toward acetate supplementation therapy for Canavan disease: glyceryl triacetate administration increases acetate, but not N-acetylaspartate, levels in brain. J Pharmacol Exp Ther 2005;315(1):297–303. Epub 2005 Jul 7. [49] Sager TN, Hansen AJ, Laursen H. Correlation between N-acetylaspartate levels and histopathologic changes in cortical infarcts of mice after middle cerebral artery occlusion. J Cereb Blood Flow Metab 2000;20(5): 780–8. [50] Sager TN, Fink-Jensen A, Hansen AJ. Transient elevation of interstitial N-acetylaspartate in reversible global brain ischemia. J Neurochem 1997;68(2):675–82.