Functions of N-acetyl-L-aspartate and N-acetyl-L-aspartylglutamate in the vertebrate brain: role in glial cell-specific signaling

N-Acetyl-L-aspartate (NAA) and its derivative N-acetylaspartylglutamate (NAAG) are major osmolytes present in the vertebrate brain. Although they are synthesized primarily in neurons, their function in these cells is unclear. In the brain, these substances undergo intercompartmental cycles in which they are released by neurons in a regulated fashion and are then rapidly hydrolyzed by catabolic enzymes associated with glial cells. Recently, the catabolic enzyme for NAA hydrolysis has been found to be expressed only in oligodendrocytes, and the catabolic enzyme for NAAG expressed only in astrocytes. These results indicate an unusual tricellular metabolic sequence for the synthesis and hydrolysis of NAAG wherein it is synthesized in neurons from NAA and L-glutamate, hydrolyzed to NAA and L-glutamate by astrocytes, and further hydrolyzed to L-aspartate and acetate by oligodendrocytes. Since the discovery that the NAA and NAAG anabolic products of neurons are specifically targeted to oligodendrocytes and astrocytes, respectively, this unique metabolic compartmentalization also suggests that these substances may play an important role in cell-specific glial signaling. In this review, it is hypothesized that a key function of NAA and NAAG in the vertebrate brain is in cell signaling and that these substances are important in the regulation of interactions of brain cells and in the establishment and maintenance of the nervous system.     

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 D₂ 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 (NAA) and N-Acetylaspartylglutamate (NAAG) Promote Growth and Inhibit Differentiation of Glioma Stem-like Cells

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

Evidence that the tri-cellular metabolism of N-acetylaspartate functions as the brain’s “operating system”: how NAA metabolism supports meaningful intercellular frequency-encoded communications

N-acetylaspartate (NAA), an acetylated derivative of l-aspartate (Asp), and N-acetylaspartylglutamate (NAAG), a derivative of NAA and l-glutamate (Glu), are synthesized by neurons in brain. However, neurons cannot catabolize either of these substances, and so their metabolism requires the participation of two other cell types. Neurons release both NAA and NAAG to extra-cellular fluid (ECF) upon stimulation, where astrocytes, the target cells for NAAG, hydrolyze it releasing NAA back into ECF, and oligodendrocytes, the target cells for NAA, hydrolyze it releasing Asp to ECF for recycling to neurons. This sequence is unique as it is the only known amino acid metabolic cycle in brain that requires three cell types for its completion. The results of this cycling are two-fold. First, neuronal metabolic water is transported to ECF for its removal from brain. Second, the rate of neuronal activity is coupled with focal hyperemia, providing stimulated neurons with the energy required for transmission of meaningful frequency-encoded messages. In this paper, it is proposed that the tri-cellular metabolism of NAA functions as the “operating system” of the brain, and is essential for normal cognitive and motor activities. Evidence in support of this hypothesis is provided by the outcomes of two human inborn errors in NAA metabolism.     

Simultaneous immunocytochemical visualization of bromodeoxyuridine and neural tissue antigens.

5'-Bromodeoxyuridine (BrdU) is a thymidine analogue which can be detected by monoclonal antibodies (MAb). We have developed a method for the simultaneous visualization of BrdU and a wide range of neural antigens in paraformaldehyde-fixed brain sections. Pregnant mice were injected intraperitoneally with a single pulse of BrdU. Young adult offspring were processed for immunocytochemistry following a double immunoperoxidase sequence. BrdU was detected using diaminobenzidine (DAB) intensified with nickel ammonium sulfate and neural antigen-containing elements were visualized with DAB alone. BrdU-positive nuclei and tissue antigen-immunoreactive cells were easily differentiated. Furthermore, double-labeled cells characterized by the presence of a black immunoreactive nucleus surrounded by a brown immunopositive cytoplasm were unambiguously recognized. Satisfactory results were obtained using either MAb or polyclonal antibodies against a variety of cell antigens, including neuropeptides, CA++ binding proteins, and cytoskeletal components of the glial cells. The method reported here permits analysis of the neurogenesis and proliferation of subsets of neurons and glial cells, identified by immunocytochemical markers.     

N-Acetylaspartylglutamate Selectively Inhibits Neuronal Responses to N-Methyl-d-Aspartic Acid In Vitro

Abstract: Canavan's disease is an autosomal recessive disorder characterized by a deficiency of aspartoacylase and accumulation of N -acetylaspartic acid (NAA), leading to a severe leukodystrophy and spongy degeneration of the brain. N -Acetylaspartylglutamate (NAAG), the presumed product of NAA, also accumulates in this disease. The endogenous dipeptide NAAG has been suggested to have low potency at NMDA receptors. Here we have tested the actions of NAAG and NAA on NMDA-evoked responses in cultured cerebellar granule cells. In differentiating granule cells grown in low-K⁺ medium, NAAG negated the survival-promoting effects of NMDA but not K⁺ depolarization. Neither NAAG nor NAA alone promoted cell survival in low-K⁺ medium. The modest trophic action of 50 µ M kainic acid in low-K⁺ medium was reinforced by the NMDA receptor antagonist dizocilpine maleate and by NAAG. In K⁺-differentiated granule cells, NAAG raised the threshold of NMDA neurotoxicity but not that of kainate. The observed activities of NAAG were overcome by excess NMDA and were not mimicked by NAA. These data raise the possibility that disruption of NMDA receptor processes by NAAG may be of pathophysiological relevance.     

Biosynthesis of NAAG by an enzyme-mediated process in rat central nervous system neurons and glia

The peptide transmitter N-acetylaspartylglutamate (NAAG) is present in millimolar concentrations in mammalian spinal cord. Data from the rat peripheral nervous system suggest that this peptide is synthesized enzymatically, a process that would be unique for mammalian neuropeptides. To test this hypothesis in the mammalian CNS, rat spinal cords were acutely isolated and used to study the incorporation of radiolabeled amino acids into NAAG. Consistent with the action of a NAAG synthetase, inhibition of protein synthesis did not affect radiolabel incorporation into NAAG. Depolarization of spinal cords stimulated incorporation of radiolabel. Biosynthesis of NAAG by cortical astrocytes in cell culture was demonstrated by tracing incorporation of [3H]-glutamate by astrocytes. In the first test of the hypothesis that NAA is an immediate precursor in NAAG biosynthesis, [3H]-NAA was incorporated into NAAG by isolated spinal cords and by cell cultures of cortical astrocytes. Data from cerebellar neurons and glia in primary culture confirmed the predominance of neuronal synthesis and glial uptake of NAA, leading to the hypothesis that while neurons synthesize NAA for NAAG biosynthesis, glia may take it up from the extracellular space. However, cortical astrocytes in serum-free low-density cell culture incorporated [3H]-aspartate into NAAG, a result indicating that under some conditions these cells may also synthesize NAA. Pre-incubation of isolated spinal cords and cultures of rat cortical astrocytes with unlabeled NAA increased [3H]-glutamate incorporation into NAAG. In contrast, [3H]-glutamine incorporation in spinal cord was not stimulated by unlabeled NAA. These results are consistent with the glutamate-glutamine cycle greatly favoring uptake of glutamine into neurons and glutamate by glia and suggest that NAA availability may be rate-limiting in the synthesis of NAAG by glia under some conditions.     

Simultaneous determination of N-acetylaspartic acid, N-acetylglutamic acid, and N-acetylaspartylglutamic acid in whole brain of 3-mercaptopropionic acid-treated rats using liquid chromatography-atmospheric pressure chemical ionization mass spectrometry

The measurement of N-acetylaspartic acid (NAA), N-acetylglutamic acid (NAG), and N-acetylaspartylglutamic acid (NAAG) in the whole brain of 3-mercaptopropionic acid (3-MPA)-treated rats has been developed using liquid chromatography-mass spectrometry with an atmospheric pressure ionization interface system. The recoveries of these compounds were 90.85 +/- 3.43% for NAA, 91.62 +/- 5.47% for NAG, and 92.29 +/- 4.44% for NAAG. The detection limits for NAA, NAG, and NAAG were 12, 15, and 20 microg/ml, respectively. After administration of 3-MPA, the concentrations of NAA, NAG, and NAAG in the whole brain over 10 min increased 177.25, 134.23, and 127.70%, respectively. These concentrations then decreased over the next 60 min. The simultaneous determination of NAA, NAG, and NAAG using this method was found to be very useful for studies of metabolism of NAA, NAG, and NAAG in biological samples.     

N-Acetyl-Aspartyl-Glutamate: Regional Levels in Rat Brain and the Effects of Brain Lesions as Determined by a New HPLC Method

Abstract: An isocratic HPLC method to measure endogenous N -acetyl-aspartyl-glutamate (NAAG) and N -acetyl-aspartate (NAA) is described. After removal of primary amines by passage of tissue extracts over AG-50 resin, the eluate was subject to HPLC anion-exchange analysis and eluted with phosphate buffer with absorbance monitored at 214 nm. The retention time for NAA was 5.6 min and for NAAG 11.4 min with a limit sensitivity of 0.1 nmol. The levels of NAA and NAAG were measured in 16 regions of rat brain and in heart and liver. NAAG was undetectable in heart and liver and exhibited 10-fold variation in concentration among brain regions; the highest levels were found in spinal cord. In contrast, low concentrations of NAA were detectable in heart and liver, and the regional distribution of NAA in brain varied only twofold. The regional distribution of NAA and NAAG correlated poorly. To assess the neuronal localization of these two compounds, the effects of selective brain lesions on their levels were examined. Decortication caused a 28% decrease in NAAG levels in the ipsi-lateral striatum while NAA decreased 38%. Kainate lesion of the striatum resulted in a 31% decrease in NAAG in the ipsilateral striatum, whereas NAA fell by 58%. Kainate lesion of the hippocampus resulted in significant decrements in NAAG and NAA in the hippocampus and septum. Transection of the spinal cord at midthorax resulted in a 51% decrease in NAAG levels immediately caudal and a 40% decrease immediately rostral to the lesion; however, NAA decreased only 30% in these areas. These results are consistent with a neuronal localization of NAAG in brain. Combined with the fact that NAAG interacts with a subpopulation of glutamate receptors, these results suggest that NAAG may serve as an excitatory neurotransmitter.     

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.     

Cell-specific immuno-probes for the brain of normal and mutant Drosophila melanogaster

Summary We have screened antibodies for immunocytochemical staining in the optic lobes of the brain of Drosophila melanogaster. Seven polyclonal antisera and five monoclonal antibodies are described that selectively and reproducibly stain individual cells and/or produce characteristic staining patterns in the neuropile. Such antisera are useful for the cellular characterization of molecular and structural brain defects in visual mutants. In the wildtype visual system we can at present separately stain the following: the entire complement of columnar T 1 neurons; a small set of presumptive serotonergic neurons; some 3000 cells that contain and synthesize -amino butyric acid (GABA); and three groups of cells that bind antibodies to Ca²⁺-binding proteins. In addition, small groups of hitherto unknown tangential cells that send fine arborizations into specific strata of the medulla, and two patterns of characteristic layers in the visual neuropile have been identified by use of monoclonal antibodies generated following immunization of mice with homogenates of the brain of Drosophila melanogaster.     

Prostaglandin synthesis by primary cultures of mouse embryo palate mesenchyme cells

Evidence is presented for a concomitant storage of α-Neo-endorphin and dynorphin immunoreactivities in neurons of the rat brain. Antisera were raised against the structurally related opioid peptides dynorphin(1–17) and α-Neo-endorphin. Both antisera were highly specific for their respective antigen. Thus, the α-Neo-endorphin antisera did not crossreact with dynorphin and the dynorphin antisera did not crossreact with α-Neo-endorphin. Both antisera were also not cross-reactive with leu-enkephalin which is contained within the sequence of both dynorphin and α-Neo-endorphin. The antisera were used for immunofluorescent staining of frozen sections through brains from rats which had been treated with colchicine 48 hours prior to death. Both antisera revealed strong and specific immunoreactivities of magnocellular neurons in the supraoptic, retrochiasmatic supraoptic and paraventricular nuclei. Neuronal fiber systems in various areas of the brain were also labeled by the two antisera. Consecutive immunostaining of the same sections, first with dynorphin antisera and — after electrophoretic elution of the antibodies — with α-Neo-endorphin antisera or vice versa, showed that immunoreactivities for the two peptides are contained within the same hypothalamic magnocellular neurons. The neuronal fiber systems for α-Neo-endorphin and dynorphin also showed a close overlap. These studies demonstrating colocalization raise the question as to whether the two peptides have a common origin from a single precursor molecule.     

The major nucleoside triphosphatase in pea (Pisum sativum L.) nuclei and in rat liver nuclei share common epitopes also present in nuclear lamins.

The major nucleoside triphosphatase (NTPase) activities in mammalian and pea (Pisum sativum L.) nuclei are associated with enzymes that are very similar both biochemically and immunochemically. The major NTPase from rat liver nuclei appears to be a 46-kD enzyme that represents the N-terminal portion of lamins A and C, two lamina proteins that apparently arise from the same gene by alternate splicing. Monoclonal antibody (MAb) G2, raised to human lamin C, both immunoprecipitates the major (47 kD) NTPase in pea nuclei and recognizes it in western blot analyses. A polyclonal antibody preparation raised to the 47-kD pea NTPase (pc480) reacts with the same lamin bands that are recognized by MAb G2 in mammalian nuclei. The pc480 antibodies also bind to the same lamin-like bands in pea nuclear envelope-matrix preparations that are recognized by G2 and three other MAbs known to bind to mammalian lamins. In immunofluorescence assays, pc480 and anti-lamin antibodies stain both cytoplasmic and nuclear antigens in plant cells, with slightly enhanced staining along the periphery of the nuclei. These results indicate that the pea and rat liver NTPases are structurally similar and that, in pea nuclei as in rat liver nuclei, the major NTPase is probably derived from a lamin precursor by proteolysis.     

Surface antigens of brain synapses: identification of minor proteins using polyclonal antisera

Antigenic proteins of brain synaptic plasma membranes (SPM) and postsynaptic densities (PSD) were characterized using antisera raised against SPM. Immunostaining of brain sections showed that the antigens were restricted to synapses, and electron microscopy revealed staining at both presynaptic terminals and PSDs. In primary brain cell cultures the antisera were also neuron-specific but the antigens were distributed throughout the entire neuronal plasma membrane, suggesting that some restrictive influence present in whole tissue is absent when neurons are grown dispersed. The antigenic proteins with which these antisera react were identified using SDS gel immunoblots. SPM and PSD differed from one another in their characteristic antigenic proteins. Comparison with amido-black stained gel blots showed that in both cases most of these did not correspond to known abundant proteins of SPM or PSDs revealed by conventional biochemical techniques. None of the antigens revealed by the polyclonal antisera were detected by any of a large series of monoclonal antibodies against SPM.     

A novel key–lock mechanism for inactivating amino acid neurotransmitters during transit across extracellular space

There are two kinds of neurotransmissions that occur in brain. One is neuron to neuron at synapses, and the other is neuron to glia via extracellular fluid (ECF), both of which are important for maintenance of proper neuronal functioning. For neuron to neuron communications, several potent amino acid neurotransmitters are used within the confines of synaptic space. However, their presence at elevated concentrations in extra-synaptic space could be detrimental to well organized neuronal functioning. The significance of the synthesis and release of N-acetylaspartylglutamate (NAAG) by neurons has long been a puzzle since glutamate (Glu) itself is the “key” that can interact with all Glu receptors on membranes of all cells. Nonetheless, neurons synthesize this acetylated dipeptide, which cannot be catabolized by neurons, and release it to ECF where its specific physiological target is the Glu metabotropic receptor 3 on the surface of astrocytes. Since Glu is excitotoxic at elevated concentrations, it is proposed that formation and release of NAAG by neurons allows large quantities of Glu to be transported in ECF without the risk of injurious excitotoxic effects. The metabolic mechanism used by neurons is a key–lock system to detoxify Glu during its intercellular transit. This is accomplished by first synthesizing N-acetylaspartate (NAA), and then joining this molecule via a peptide bond to Glu. In this paper, a hypothesis is presented that neurons synthesize a variety of relatively nontoxic peptides and peptide derivatives, including NAA, NAAG, homocarnosine (γ-aminobutyrylhistidine) and carnosine (β-alanylhistidine) from potent excitatory and inhibitory amino acids for the purpose of releasing them to ECF to function as cell-specific neuron-to-glia neurotransmitters.     

Absence of pituitary prolactin epitopes in immunoreactive prolactin of rat brain.

Immunoreactive prolactin (ir-PRL) in rat brain has been consistently documented. However, the identity of this ir-PRL is controversial. Ir-PRL is defined by its ability to bind to PRL antibodies. All previous studies of brain ir-PRL have used polyclonal antibodies, at least one of which apparently crossreacts with a portion of the proopiomelanocortin molecule. To begin to define the epitopes comprising ir-PRL in the brain, we utilized two monoclonal antibodies (MAb) that recognize pituitary PRL in a variety of species, including rat. Immunocytochemistry was performed on rat brains and pituitary glands using two monoclonal and one polyclonal PRL antibody. Although both MAb immunostained lactotrophs of the rat pituitary gland, neither antibody immunostained cell bodies or neuronal processes in the brain. However, the polyclonal antiserum immunostained lactotrophs and a system of neuronal cell bodies and processes in the brain. Thus, epitopes found in pituitary PRL from several species are not found in ir-PRL in rat brain.     

Characterization by immunocytochemistry of ionic channels in Helix aspersa suboesophageal brain ganglia neurons

The aim of this work was to characterize several ionic channels in nervous cells of the suboesophageal visceral, left and right parietal, and left and right pleural brain ganglia complex of the snail Helix aspersa by immunocytochemistry. We have studied the immunostaining reaction for a wide panel of eleven polyclonal antibodies raised against mammal antigens as follows: voltage-gated-Na+ channel; voltage-gated-delayed-rectifier-K+ channel; SK2-small-conductance-Ca2+-dependent-K+ channel apamin sensitive; SK3 potassium channel; charybdotoxin-sensitive voltage-dependent potassium channel; BKCa-maxi-conductance-Ca2+-dependent-K+ channel; hyperpolarization-activated cyclic nucleotide-gated potassium channel 4; G-protein-activated inwardly rectifying potassium channel GIRK2 and voltage-gated-calcium of L, N and P/Q type channels. Our results show positive reaction in neurons, but neither in glia cells nor in processes in the Helix suboesophageal ganglia. Our results suggest the occurrence of molecules in Helix neurons sharing antigenic determinants with mammal ionic channels. The reaction density and distribution of immunoreactive staining within neurons is specific for each one of the antisera tested. The studies of co-localization of immunoreaction, on alternate serial sections of the anterior right parietal ganglion, have shown for several recognized mapped neurons that they can simultaneously be expressed among two and seven different ionic protein channels. These results are considered a key structural support for the interpretation of Helix aspersa neuron electrophysiological activity.     

Use of a monoclonal rat anti-mouse Ig light chain (RAMOL-1) antibody reduces background binding in immunohistochemical and fluorescent antibody analysis

Rat monoclonal antibodies (MAb) directed to mouse Ig heavy and light chain determinants were produced. A rat anti-mouse light chain MAb (RAMOL-1) which bound to all (24/24) mouse Ig of the kappa light chain type and with varying strength to 4/4 lambda light chain-bearing Ig was evaluated as a general secondary reagent, together with two MAb that bound to the heavy chain of mouse IgG. They were conjugated with biotin or FITC and used in immunohistochemical and immunofluorescence assays to detect mouse monoclonal antibodies binding to antigens expressed in rat and human tissues and cells. As compared to commercially available polyclonal reagents, RAMOL-1 gave higher staining contrast by showing lower background staining and equal or higher staining of the primary MAb tested. This was a result of two main effects. First, crossreactivity with endogenous Ig and tissue type-specific determinants was eliminated. With polyclonal anti-mouse Ig reagents, binding to endogenous Ig was noted in vascular spaces and on Ig-bearing cells, and to rat gastric mucosa and epithelial tumor tissue in frozen tissue sections, even when diluted in high concentrations of serum homologous to the tissue. Second, binding of the secondary reagent was reduced to cells and tissues prone to have high nonspecific binding capability, such as monocytes/macrophages and formalin-fixed, paraffin-embedded tissue. Owing to unlimited and reproducible access to this homogeneous reagent, RAMOL-1 is used as second antibody to standardize the procedure used for immunohistochemical grading of human malignant tumors by determination of blood group antigen expression detected with mouse MAb.     

Molecular Characterization of N-Acetylaspartylglutamate Synthetase

The dipeptide N-acetylaspartyl-glutamate (NAAG) is an abundant neuropeptide in the mammalian brain. Despite this fact, its physiological role is poorly understood. NAAG is synthesized by a NAAG synthetase catalyzing the ATP-dependent condensation of N-acetylaspartate and glutamate. In vitro NAAG synthetase activity has not been described, and the enzyme has not been purified. Using a bioinformatics approach we identified a putative dipeptide synthetase specifically expressed in the nervous system. Expression of the gene, which we named NAAGS (for NAAG synthetase) was sufficient to induce NAAG synthesis in primary astrocytes or CHO-K1 and HEK-293T cells when they coexpressed the NAA transporter NaDC3. Furthermore, coexpression of NAAGS and the recently identified N-acetylaspartate (NAA) synthase, Nat8l, in CHO-K1 or HEK-293T cells was sufficient to enable these cells to synthesize NAAG. Identity of the reaction product of NAAGS was confirmed by HPLC and electrospray ionization tandem mass spectrometry (ESI-MS). High expression levels of NAAGS were restricted to the brain, spinal cord, and testis. Taken together our results strongly suggest that the identified gene encodes a NAAG synthetase. Its identification will enable further studies to examine the role of this abundant neuropeptide in the vertebrate nervous system.     

Immunohistochemistry and Biosynthesis of N-Acetylaspartylglutamate in Spinal Sensory Ganglia

N-Acetylaspartylglutamate (NAAG) is a nervous system-specific dipeptide which has been implicated in chemical neurotransmission. Antisera were prepared against NAAG in order to study its cellular distribution. When these antisera were applied to tissue sections of rat spinal sensory ganglia, NAAG-like immunoreactivity was detected within a subpopulation of relatively large neuronal cell bodies in cervical, lumbar, and thoracic ganglia. In order to confirm the presence of NAAG within these neurons, the dipeptide was extracted and purified from spinal ganglia using high-performance liquid chromatography and its composition confirmed by amino acid analysis. Further, the biosynthesis of NAAG was studied in vitro by following the incorporation of either [3H]glutamine or [3H]glutamate into the glutamate residue of the purified dipeptide. [3H]Aspartate was not incorporated efficiently into NAAG under these conditions, suggesting a precursor role for the large N-acetylaspartate pool. The incorporation of radiolabeled amino acids into newly synthesized NAAG by spinal sensory ganglia was not inhibited by incubation of the cells with anisomycin or cycloheximide at concentrations which significantly inhibited protein synthesis. These data suggest that NAAG is present in a subpopulation of primary afferent spinal neurons and that its biosynthesis is mediated by a dipeptide synthetase.