Localization and Synaptic Release of N-acetylaspartylglutamate in the Chick Retina and Optic Tectum

Spatial distribution of metabolites in the retina and its relevance to studies of metabolic retinal disorders

INTRODUCTION

The primate retina has evolved regional specialisations for specific visual functions. The macula is specialised towards high acuity vision and is an area that contains an increased density of cone photoreceptors and signal processing neurons. Different regions in the retina display unique susceptibility to pathology, with many retinal diseases primarily affecting the macula.

OBJECTIVES

To better understand the properties of different retinal areas we studied the differential distribution of metabolites across the retina.

METHODS

We conducted an untargeted metabolomics analysis on full-thickness punches from three different regions (macula, temporal peri-macula and periphery) of healthy primate retina.

RESULTS

Nearly half of all metabolites identified showed differential abundance in at least one comparison between the three regions. Furthermore, mapping metabolomics results from macula-specific eye diseases onto our region-specific metabolite distributions revealed differential abundance defining systemic metabolic dysregulations that were region specific.

CONCLUSIONS

The unique metabolic phenotype of different retinal regions is likely due to the differential distribution of different cell types in these regions reflecting the specific metabolic requirements of each cell type. Our results may help to better understand the pathobiology of retinal diseases with region specificity.

Evidence of NAAG-family tripeptide NAAG2 in the Drosophila nervous system

N-acetylaspartylglutamate (NAAG) is a common neurotransmitter in the mammalian nervous system; however, it has never been reported in the nervous system of the fruit fly, Drosophila melanogaster. Using antiserum against NAAG, we localized NAAG-like immunoreactivity to neurons in the ventral nerve cord and to type Is glutamatergic nerve terminals in larval neuromuscular junctions. Using liquid chromatography tandem mass spectrometry (LC-MS), we failed to find NAAG but found the related peptide N-acetylaspartylglutamylglutamate (NAAG₂ ) in Drosophila CNS and body wall tissue. This is the first report of any NAAG-family peptide in the nervous system of Drosophila and is also the first report of NAAG₂ being present in a much higher concentration than NAAG in the nervous system of any species. Thus, the larval fruit fly presents an interesting model for the study of the functional role of NAAG₂ of which very little is known-especially in the absence of an abundance of NAAG.

Riluzole decreases synthesis of N-acetylaspartate and N-acetylaspartylglutamate in SH-SY5Y human neuroblastoma cells

N-acetylaspartate (NAA) is present at very high concentrations in the brain and is used as a non-invasive marker of neuronal viability in magnetic resonance spectroscopy. N-acetylaspartylglutamate (NAAG) is an acetylated dipeptide formed from NAA, and may be an agonist of the mGluR3 receptor. Both NAA and NAAG are synthesized primarily in neurons. Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disorder resulting in motor neuron death, and progressive paralysis. Levels of both NAA and NAAG are reported to be decreased in ALS. Riluzole is a glutamatergic modulating agent used to treat ALS, but there are conflicting results in the literature concerning the recovery of NAA after riluzole treatment. We studied the effects of riluzole on the biosynthesis of both NAA and NAAG in SH-SY5Y human neuroblastoma cells. We used two methodologies to examine the effect; one involving radiolabel incorporation from corresponding substrates into NAA and NAAG, and the other involving the measurement of endogenous NAA and NAAG levels using HPLC. We show that riluzole treatment, which decreases glutamatergic neuronal excitation, decreases the synthesis and levels of both NAA and NAAG in SH-SY5Y cells in a dose and time dependant manner. These results suggest that the synthesis of NAA and NAAG may be coupled to glutamatergic neurotransmission, and further investigations along these lines are warranted.

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 glutamate carboxypeptidase II hydrolysis of N ‐acetylaspartylglutamate (NAAG) in crayfish nervous tissue is mediated by glial glutamate and acetylcholine receptors

Glutamate carboxypeptidase II (GCPII), a glial ectoenzyme, is responsible for N-acetylaspartylglutamate (NAAG) hydrolysis. Its regulation in crayfish nervous tissue was investigated by examining uptake of [3H]glutamate derived from N-acetylaspartyl-[3H]glutamate ([3H]NAAG) to measure GCPII activity. Electrical stimulation (100 Hz, 10 min) during 30 min incubation with [3H]NAAG increased tissue [3H]glutamate tenfold. This was prevented by 2-(phosphonomethyl)-pentanedioic acid (2-PMPA), a GCPII inhibitor, suggesting that stimulation increased the hydrolysis of [3H]NAAG and metabolic recycling of [3H]glutamate. Antagonists of glial group II metabotropic glutamate receptors (mGLURII), NMDA receptors and acetylcholine (ACh) receptors that mediate axon-glia signaling in crayfish nerve fibers decreased the effect of stimulation by 58-83%, suggesting that glial receptor activation leads to stimulation of GCPII activity. In combination, they reduced [3H]NAAG hydrolysis during stimulation to unstimulated control levels. Agonist stimulation of mGLURII mimicked the effect of electrical stimulation, and was prevented by antagonists of GCPII or mGLURII. Raising extracellular K+ to three times the normal level stimulated [3H]NAAG release and GCPII activity. These effects were also blocked by antagonists of GCPII and mGLUR(II). No receptor antagonist or agonist tested or 2-PMPA affected uptake of [3H]glutamate. We conclude that NAAG released from stimulated nerve fibers activates its own hydrolysis via stimulation of GCPII activity mediated through glial mGLURII, NMDA and ACh receptors.

SH-SY5Y neuroblastoma cells: a model system for studying biosynthesis of NAAG

N-Acetylaspartylglutamate (NAAG) is a neuropeptide that is thought to modulate neurotransmitter release through pre-synaptic mGluR3 receptors. Despite years of research into NAAG biochemistry, almost nothing is known about NAAG biosynthesis. To date, NAAG biosynthesis has only been demonstrated conclusively in explanted animal neural tissues, including frog retina, rat dorsal root ganglia and crayfish nerve cord, but not in human cells or tissues. We show here that a human neuroblastoma cell line, SH-SY5Y, provides a good model system for the study of NAAG biosynthesis. Radiolabled NAAG synthesis occurred using both L-[3H]glutamic acid and L-[3H]glutamine as precursors, with glutamine being the preferred substrate. Differentiation of SH-SY5Y cells with retinoic acid resulted in decreased radiolabel incorporation into NAAG, whereas differentiation with nerve growth factor did not affect radiolabel incorporation.

Reductions in N-acetylaspartylglutamate and the 67 kDa form of glutamic acid decarboxylase immunoreactivities in the visual system of albino and pigmented rats after optic nerve transections

This study compares the immunohistochemical distributions of N-acetylaspartylglutamate (NAAG) and the large isoform of the gamma-aminobutyric acid (GABA)-synthesizing enzyme glutamic acid decarboxylase (GAD(67)) in the visual system of albino and pigmented rats. Most retinal ganglion cells and their axons were strongly immunoreactive for NAAG, whereas GAD(67) immunoreactivity was very sparse in these cells and projections. In retinorecipient zones, NAAG and GAD(67) immunoreactivities occurred in distinct populations of neurons and in dense networks of strongly immunoreactive fibers and synapses. Dual-labeling immunohistochemistry indicated that principal neurons were stained for NAAG, whereas local interneurons were stained for GAD(67). In contrast to the distribution observed in retinorecipient zones, most or all neurons were doubly stained for NAAG and GAD(67) in the thalamic reticular nucleus. Ten days after unilateral optic nerve transection, NAAG-immunoreactive fibers and synapses were substantially reduced in all contralateral retinal terminal zones. The posttransection pattern of NAAG-immunoreactive synaptic loss demarcated the contralateral and ipsilateral divisions of the retinal projections. In addition, an apparent transynaptic reduction in GAD(67) immunoreactivity was observed in some deafferented areas, such as the lateral geniculate. These findings suggest a complicated picture in which NAAG and GABA are segregated in distinct neuronal populations in primary visual targets, yet they are colocalized in neurons of the thalamic reticular nucleus. This is consistent with NAAG acting as a neurotransmitter release modulator that is coreleased with a variety of classical transmitters in specific neural pathways.

Deletion of the glutamate carboxypeptidase II gene in mice reveals a second enzyme activity that hydrolyzes N-acetylaspartylglutamate

Glutamate carboxypeptidase II (GCPII, EC 3.14.17.21) is a membrane-bound enzyme found on the extracellular face ofglia. The gene for this enzyme is designated FOLH1 in humans and Folh1 in mice. This enzyme has been proposed to be responsible for inactivation of the neurotransmitter N-acetylaspartylglutamate (NAAG) following synaptic release. Mice harboring a disruption of the gene for GCPII/Folh1 were generated by inserting into the genome a targeting cassette in which the intron-exon boundary sequences of exons 1 and 2 were removed and stop codons were inserted in exons 1 and 2. Messenger RNA for GCPII was not detected by northern blotting or RT-PCR analysis of RNA from the brains of -/- mutant mice nor was GCPII protein detected on western blots of this tissue. These GCPII null mutant mice developed normally to adulthood and exhibited a normal range of neurologic responses and behaviors including mating, open field activity and retention of position in rotorod tests. No significant differences were observed among responses of wild type, heterozygous mutant and homozygous mutant mice on tail flick and hot plate latency tests. Glutamate, NAAG and mRNA for metabotropic glutamate receptor type 3 levels were not significantly altered in response to the deletion of glutamate carboxypeptidase II. A novel membrane-bound NAAG peptidase activity was discovered in brain, spinal cord and kidney of the GCPII knock out mice. The kinetic values for brain NAAG peptidase activity in the wild type and GCPII nullmutant were Vmax = 45 and 3 pmol/mg/min and Km = 2650 nm and 2494 nm, respectively. With the exception of magnesium and copper, this novel peptidase activity had a similar requirement for metal ions as GCPII. Two potent inhibitors of GCPII, 4,4'-phosphinicobis-(butane-1,3 dicarboxilic acid) (FN6) and 2-(phosphonomethyl)pentanedioic acid (2-PMPA) inhibited the residual activity. The IC50 value for 2-PMPA was about 1 nm for wild-type brain membrane NAAG peptidase activity consistent with its activity against cloned ratand human GCPII, and 88 nm for the activity in brain membranes of the null mutants.

Synthesis and release of N-acetylaspartylglutamate (NAAG) by crayfish nerve fibers: implications for axon-glia signaling

Early physiological and pharmacological studies of crayfish and squid giant nerve fibers suggested that glutamate released from the axon during action potential generation initiates metabolic and electrical responses of periaxonal glia. However, more recent investigations in our laboratories suggest that N-acetylaspartylglutamate (NAAG) may be the released agent active at the glial cell membrane. The investigation described in this paper focused on NAAG metabolism and release, and its contribution to the appearance of glutamate extracellularly. Axoplasm and periaxonal glial cell cytoplasm collected from medial giant nerve fibers (MGNFs) incubated with radiolabeled L-glutamate contained radiolabeled glutamate, glutamine, NAAG, aspartate, and GABA. Total radiolabel release was not altered by electrical stimulation of nerve cord loaded with [(14)C]glutamate by bath application or loaded with [(14)C]glutamate, [(3)H]-D-aspartate or [(3)H]NAAG by axonal injection. However, when radiolabeled glutamate was used for bath loading, radiolabel distribution among glutamate and its metabolic products in the superfusate was changed by stimulation. NAAG was the largest fraction, accounting for approximately 50% of the total recovered radiolabel in control conditions. The stimulated increase in radioactive NAAG in the superfusate coincided with its virtual clearance from the medial giant axon (MGA). A small, stimulation-induced increase in radiolabeled glutamate in the superfusate was detected only when a glutamate uptake inhibitor was present. The increase in [(3)H]glutamate in the superfusion solution of nerve incubated with [(3)H]NAAG was reduced when beta-NAAG, a competitive glutamate carboxypeptidase II (GCP II) inhibitor, was present.Overall, these results suggest that glutamate is metabolized to NAAG in the giant axon and its periaxonal glia and that, upon stimulation, NAAG is released from the axon and converted in part to glutamate by GCP II. A quisqualate- and beta-NAAG-sensitive GCP II activity was detected in nerve cord homogenates. These results, together with those in the accompanying paper demonstrating that NAAG can activate a glial electrophysiological response comparable to that initiated by glutamate, implicate NAAG as a probable mediator of interactions between the MGA and its periaxonal glia.

N-Acetylaspartylglutamate: the most abundant peptide neurotransmitter in the mammalian central nervous system

In the progress of science, as in life, timing is important. The acidic dipeptide, N-acetylaspartylglutamate (NAAG), was discovered in the mammalian nervous system in 1965, but initially was not considered to be a neurotransmitter candidate. In the mid-1980s, a few laboratories revisited the question of NAAG's role in the nervous system and pursued hypotheses regarding its function that ranged from a precursor for the transmitter pool of glutamate to a direct role as a peptide transmitter. Since that time, NAAG has been tested against nearly all of the established criteria for identification of a neurotransmitter. It successfully meets each of these tests, including a concentrated presence in neurons and synaptic vesicles, release from axon endings in a calcium-dependent manner following initiation of action potentials, and extracellular hydrolysis by membrane-bound peptidase activity. NAAG is the most prevalent and widely distributed neuropeptide in the mammalian nervous system. NAAG activates NMDA receptors with a low potency that may vary among receptor subtypes, and it is a highly selective agonist at the type 3 metabotropic glutamate receptor (mGluR3). Acting through this receptor, NAAG reduces cyclic AMP levels, decreases voltage-dependent calcium conductance, suppresses excitotoxicity, influences long-term potentiation and depression, regulates GABA(A) receptor subunit expression, and inhibits synaptic release of GABA from cortical neurons. Cloning of peptidase activities against NAAG provides opportunities to study the cellular and molecular mechanisms by which synaptic NAAG peptidase activity is controlled. Given the codistribution of this peptide with a spectrum of traditional transmitters and its ability to activate mGluR3, we speculate that one role for NAAG following synaptic release is the activation of metabotropic autoreceptors that inhibit subsequent transmitter release. A second role is the production of extracellular glutamate following NAAG hydrolysis.

N-acetylaspartylglutamate activates cyclic AMP-coupled metabotropic glutamate receptors in cerebellar astrocytes

N-Acetylaspartylglutamate (NAAG) is the most prevalent peptide in the mammalian nervous system. NAAG meets the traditional criteria of a neurotransmitter, including localization in synaptic vesicles, depolarization-induced release, low potency activation of some N-methyl-D-aspartate receptors, and highly selective activation of a cAMP-coupled metabotropic glutamate receptor (mGluR) with potency approaching that of glutamate. The peptide is present in cultured cortical glia in high concentration and is hydrolyzed by cell surface peptidase activity. In the present work, we tested the hypothesis that NAAG selectively activates a subclass of metabotropic receptors on cultured rat cerebellar glia, primarily astrocytes. These glial cells express mRNA for mGluR subtypes 1, 3, 4, 5, 6, 7, and 8. We were not able to detect message for mGluR2 in these cells using the reverse transcriptase-polymerase chain reaction. Cerebellar glia responded to NAAG, glutamate, and trans-ACPD with a decrease in forskolin-stimulated cAMP formation. AP4, an agonist of the group III receptors mGluR4, mGluR6, mGluR7, and mGluR8, had little or no effect on stimulated cAMP levels. Treatment with low micromolar NAAG significantly increased uptake of radioactive thymidine by cultured astrocytes through activation of metabotropic glutamate receptors. Antagonists of group II mGluRs prevented the decrease in cAMP and the increase in uptake of thymidine by NAAG. Cultured cerebellar astrocytes expressed 20 pmol NAAG per mg protein, a value that is at least 30-fold lower than that expressed by mixed glial cultures prepared from mouse cortex. We conclude that cerebellar astrocytes respond to NAAG via the mGluR3 receptor and that the peptide may selectively activate this receptor subtype in astrocytes following release from neurons or glia.

Electrophysiological characteristics of the proton-coupled peptide transporter PEPT2 cloned from rat brain

We have cloned a peptide transporter from rat brain and found it to be identical to rat kidney PEPT2. In the present study we characterize the transport function of the rat brain PEPT2, with special emphasis on electrophysiological properties and interaction with N-acetyl-L-aspartyl-L-glutamate (NAAG). When heterologously expressed in HeLa cells and in SK-N-SH cells, PEPT2 transports several dipeptides but not free amino acids in the presence of a proton gradient. NAAG competes with other peptides for the PEPT2-mediated transport process. When PEPT2 is expressed in Xenopus laevis oocytes, substrate-induced inward currents are detectable with dipeptides of differing charge in the presence of a proton gradient. Proton activation kinetics are similar for differently charged peptides. NAAG is a transportable substrate for PEPT2, as evidenced by NAAG-induced currents. The Hill coefficient for protons for the activation of the transport of differently charged peptides, including NAAG, is 1. Although the peptide-to-proton stoichiometry for negatively charged peptides is 1, the transport nonetheless is associated with transfer of positive charge into the oocyte, as indicated by peptide-induced inward currents.

Neuroprotective activity of N-acetylaspartylglutamate in cultured cortical cells

The endogenous dipeptide, alpha-N-acetylaspartylglutamate behaves as a partial agonist of N-methyl-D-aspartate receptors, but can also activate metabotropic glutamate receptors, with a high degree of selectivity for the metabotropic glutamate receptor 3 subtype. Knowing that agonists of group-II metabotropic glutamate receptors (i.e. of mGlu2 and -3 receptors) are neuroprotective, we have examined the neuroprotective activity of alpha-N-acetylaspartylglutamate in mixed cultures of mouse cortical cells exposed to a toxic pulse with N-methyl-D-aspartate. Alpha-N-acetylaspartylglutamate co-applied with N-methyl-D-aspartate was neuroprotective, but its action was insensitive to the selective group-II metabotropic glutamate receptor antagonist, ethylglutamate. Protection was instead antagonized by ethylglutamate when alpha-N-acetylaspartylglutamate was applied to the cultures immediately after the N-methyl-D-aspartate pulse, a condition in which there was no direct competition between alpha-N-acetylaspartylglutamate and N-methyl-D-aspartate at the level of N-methyl-D-aspartate receptors. alpha-N-acetylaspartylglutamate was highly neuroprotective when transiently applied to pure cultures of cortical astrocytes and the conditioned medium, collected 20 h later, was transferred to sister mixed cultures challenged with N-methyl-D-aspartate. This particular form of neuroprotection was attenuated or abolished when astrocytes where exposed to alpha-N-acetylaspartylglutamate in the presence of the group-II metabotropic glutamate receptor antagonists ethylglutamate or (2S, 1'S,2'S,3'R)-2-(2'-carboxy-3'-phenylcyclopropyl)glycine, but not in the presence of the N-methyl-D-aspartate receptor antagonist, D-2-amino-5-phosphonopentanoate. These results indicate that alpha-N-acetylaspartylglutamate induces neuroprotective effects in culture, which are mediated, at least in part, by the activation of glial metabotropic glutamate receptor 3 receptors.

Light and electron microscopic immunohistochemical localization of N-acetylaspartylglutamate (NAAG) in the olivocerebellar pathway of the rat

The inferior olive (IO) is the sole contributor of climbing fibers (CF) to the Purkinje cells of the cerebellar cortex. Although the anatomy and the connectivity between the IO and the cerebellum have been well established, there is still controversy regarding the neurotransmitter systems mediating olivocerebellar projections. The excitatory amino acids, glutamate (Glu) and aspartate (Asp), have both been considered as neurotransmitter candidates of olivocerebellar projections in the rat. More recently N-acetylaspartylglutamate (NAAG) has also been proposed as a transmitter of cerebellar climbing fibers based on biochemical and electrophysiological data. The aim of the present study was to determine whether NAAG immunoreactivity is present in the IO and CF at the light and electron microscopic levels and to quantitate the amount of immunogold labeling in olivary neurons and climbing fiber terminals containing this dipeptide. A polyclonal antisera against NAAG was utilized with a peroxidase-labeled avidin-biotin procedure to demonstrate these immunoreactive neurons in the IO at the light microscopic level. Approximately 45% of olivary neurons display NAAG-like immunoreactivity, and their distribution is unevenly clustered throughout the inferior olive. Using postembedding immunogold electron microscopy in combination with quantitative procedures, we found the highest densities of gold particles in the axonal terminals synapsing on olivary neurons (101.0 particles/microns2), in CF terminals (96.3 particles/microns2), and in some mossy fiber terminals (101.0 particles/microns2). Approximately half of the climbing fiber terminals examined were unlabeled. Moderate labeling occurred in CF axons (70.8 particles/microns2), while IO neuronal perikarya were lightly but significantly labeled (41.6 particles/microns2). The localization of NAAG in the subset of cerebellar climbing fiber terminals provides anatomical support for the hypothesis that NAAG may serve as a neurotransmitter/neuromodulator candidate in the olivocerebellar pathway.

Enrichment of glutamate-like immunoreactivity in the retinotectal terminals of the viper Vipera aspis: an electron microscope quantitative immunogold study

A post-embedding immunogold study was carried out to estimate the immunoreactivity to glutamate in retinal terminals, P axon terminals and dendrites containing synaptic vesicles in the superficial layers of the optic tectum of Vipera. Retinal terminals, identified following either intraocular injection of tritiated proline, horseradish peroxidase (HRP) or short-term survivals after retinal ablation, were observed to be highly glutamate-immunoreactive. A detailed quantitative analysis showed that about 50% of glutamate immunoreactivity was localized over the synaptic vesicles, 35.8% over mitochondria and 14.2% over the axoplasmic matrix. The close association of immunoreactivity with the synaptic vesicles could indicate that Vipera retino-tectal terminals may use glutamate as their neurotransmitter. P axon terminals and dendrites containing synaptic vesicles, strongly gamma-aminobutyric (GABA)-immunoreactive, were shown to be also moderately glutamate-immunoreactive, but two to three times less than retinal terminals. Moreover, in P axon terminals, the glutamate immunoreactivity was denser over mitochondria than over synaptic vesicles, possibly reflecting the 'metabolic' pool of glutamate, which serves as a precursor in the formation of GABA.

N-acetylaspartylglutamate immunoreactivity in human retina

The acidic dipeptide N-acetylaspartylglutamate (NAAG), which satisfies many of the criteria for a neurotransmitter, was identified immunohistochemically within two human retinae. We observed NAAG immunoreactivity in retinal ganglion cells, their dendrites in the inner plexiform layer, and their axons in the optic nerve fiber layer. The vast majority of ganglion cells were stained, including displaced ganglion cells, ganglion cells of different sizes, and those whose dendrites arborized in the inner and outer sublaminae of the inner plexiform layer, that is, presumed On- and Off- cells. The sizes of labeled and unlabeled cells in the ganglion cell layer, as measured in counterstained material, suggest that the unlabeled cells consist primarily or only of displaced amacrine cells. We also saw immunoreactivity in small cells along the inner margin of the inner nuclear layer, presumably amacrine cells, and in small cells with little cytoplasm in the inner plexiform and ganglion cell layers, presumably displaced amacrine cells. These results are consistent with a role for NAAG in the transmission of visual information from the retina to the rest of the brain. Further, they are similar to those reported previously in rat, cat and monkey, thus demonstrating the relevance of previous studies to humans.

Changes in the pattern of glutamate-like immunoreactivity in rat superior colliculus following retinal and visual cortical lesions

We have investigated the pattern of glutamate-like immunoreactivity in the superficial layers of the rat superior colliculus by means of postembedding immunocytochemical methods for light and electron microscopy. At the light microscopic level, labelling was faintly to moderately intense in most perikarya of the stratum zonale, stratum griseum superficiale and stratum opticum. Furthermore, strong glutamate-immunoreactive terminal-like elements were accumulated most densely in stratum zonale, stratum griseum superficiale and stratum opticum. At the electron microscopic level, a postembedding immunogold method revealed that the vast majority of those labelled elements corresponded to retinal and visual cortical terminals. These profiles were about twice as heavily labelled as their postsynaptic partners. To determine the contribution of retinal and cortical afferents to the pattern of glutamate-like immunoreactivity, rats were subjected to right retinal ablation, left cortical ablation or combined right retinal and left cortical ablations. After retinal ablation, strongly labelled perikarya were observed in the retinorecipient layers. Furthermore, a prominent loss of glutamate-immunoreactive terminal-like elements occurred in stratum zonale and stratum griseum superficiale. Ipsilateral superior colliculus to cortical ablation exhibited subtle changes characterized by a moderate increase in perikaryal immunostaining in stratum zonale, stratum griseum superficiale and stratum opticum and by an apparent discrete reduction of labelled dots in stratum griseum superficiale and stratum opticum. In cases with combined lesions, strongly immunoreactive cell bodies and dendrites were accompanied by a massive disappearance of labelled terminal-like elements in stratum zonale, stratum griseum superficiale and stratum opticum. The effect of retinal and visual cortical ablations on the pattern of glutamate-like immunoreactivity suggests that these afferents are the major sources for glutamate-immunoreactive terminals in the rat superior colliculus. In addition, these findings provide further evidence for glutamate as neurotransmitter in the visual pathways studied.

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

Contradictory immunohistochemical data have been reported on the localization of N-acetylaspartylglutamate in the rat forebrain, using different carbodiimide fixation protocols and antibody purification methods. In one case, N-acetylaspartylglutamate immunoreactivity was observed in apparent interneurons throughout all allocortical and isocortical regions, suggesting possible colocalization with GABA. In another case, strong immunoreactivity was observed in numerous pyramidal cells in neocortex and hippocampus, suggesting colocalization with glutamate or aspartate. Reconciling these disparate findings is crucial to understanding the role of N-acetylaspartylglutamate in nervous system function. Antibodies to N-acetylaspartylglutamate and a structurally related molecule, N-acetylaspartate, were purified in stages, and their cross-reactivities with protein conjugates of N-acetylaspartylglutamate and N-acetylaspartate were monitored at each stage by solid-phase immunoassay. Reduction of the cross-reactivity of the anti-N-acetylaspartylglutamate antibodies of N-acetylaspartate-protein conjugates to about 1% eliminated significant staining of most pyramidal neurons in the rat forebrain. Utilizing highly purified antibodies, the distributions of N-acetylaspartylglutamate and N-acetylaspartate were examined in several major telencephalic and diencephalic regions of the rat, and were found to be distinct. N-acetylaspartylglutamate-immunoreactivity was observed in specific neuronal populations, including many groups thought to use GABA as a neurotransmitter. Among these were the globus pallidus, ventral pallidum, entopeducular nucleus, thalamic reticular nucleus, and scattered non-pyramidal neurons in all layers of isocortex and allocortex. N-acetylaspartate-immunoreactivity was more broadly distributed than N-acetylaspartylglutamate-immunoreactivity in the rat forebrain, appearing strongest in many pyramidal neurons. Although N-acetylaspartate-immunoreactivity was found in most neurons, it exhibited a great range of intensities between different neuronal types.

Comparative distribution of N-acetylaspartylglutamate and GAD67 in the cerebellum and precerebellar nuclei of the rat utilizing enhanced carbodiimide fixation and immunohistochemistry

The most prevalent peptide in the nervous system, N-acetylaspartylglutamate (NAAG), specifically activates N-methyl D-aspartate (NMDA) receptors and a subclass of metabotropic glutamate receptors. One action of this peptide may be to modulate the release of other neurotransmitters, including gamma-aminobutyric acid (GABA). The present study describes the cellular distribution of NAAG, relative to GABA, in the cerebellum and precerebellar nuclei as a foundation for further physiological investigations. Numerous cells of origin for mossy fibers, including many of the larger neurons of the pontine nuclei, lateral reticular nuclei, vestibular nuclei, reticulotegmental nuclei, and spinal grey, were moderately to strongly stained for NAAG. Many NAAG-labeled fibers were clearly visible in the cerebellar peduncles and central white matter. Mossy fibers and mossy endings were among the most prominent NAAG-immunoreactive elements in the cerebellar cortex. Most neurons in the inferior olive were not stained for NAAG, and only sparse, lightly immunoreactive, climbing fiber-like endings could be identified in restricted regions of the cortical molecular layer. Purkinje neurons ranged from nonreactive to moderately positive, with the great majority being unstained. Cerebellar granule cells did not exhibit any NAAG immunoreactivity. A population of neurons in the deep cerebellar nuclei was highly immunoreactive for NAAG. Additionally, many neurons of the red nucleus were intensely stained for NAAG. Comparisons with staining for the 67 kD form of glutamic acid decarboxylase in serial sections revealed complementary distributions, with NAAG in excitatory pathways and cell groups, and glutamic acid decarboxylase in inhibitory systems. These findings suggest a significant functional involvement of NAAG in the excitatory afferent and efferent projection systems and provide an anatomical basis for investigations into the interactions of NAAG and GABA in the cerebellum.

The regional distribution of N-acetylaspartylglutamate (NAAG) and peptidase activity against NAAG in the rat nervous system

N-Acetylaspartylglutamate (NAAG), a prevalent peptide in the vertebrate nervous system, may be hydrolyzed by extracellular peptidase activity to produce glutamate and N-acetylaspartate. Hydrolysis can be viewed as both inactivating the peptide after synaptic release and increasing synaptic levels of ambient glutamate. To test the hypothesis that NAAG and the peptidase activity that hydrolyzes it coexist as a unique, two-stage system of chemical neurotransmission, 50 discrete regions of the rat CNS were microdissected for assay. In each microregion, the concentration of NAAG was determined by radioimmunoassay and the peptidase activity was assayed using tritiated peptide as substrate. The NAAG concentration ranged from 2.4 nmol/mg of soluble protein in median eminence to 64 in thoracic spinal cord. Peptidase activity against NAAG ranged from 54 pmol of glutamate produced per milligram of membrane protein per minute in median eminence to 148 in superior colliculus. A linear relationship was observed between NAAG peptidase and NAAG concentration in 46 of the 50 areas, with a slope of 2.26 and a correlation coefficient of 0.45. These data support the hypothesis that hydrolysis of NAAG to glutamate and N-acetylaspartate is a consistent aspect of the physiology and metabolism of this peptide after synaptic release. The ratio of peptide concentration to peptidase activity was > 0.3 in the following four areas: ventrolateral medulla and reticular formation where the peptide is concentrated in axons of passage, thoracic spinal cord, where NAAG is concentrated in ascending sensory tracts as well as motoneuron cell bodies, and ventroposterior thalamic nucleus.

N-acetylaspartylglutamate inhibits forskolin-stimulated cyclic AMP levels via a metabotropic glutamate receptor in cultured cerebellar granule cells

The neuronal dipeptide N-acetylaspartylglutamate (NAAG) fulfills several of the criteria for classification as a neurotransmitter including localization in synaptic vesicles, calcium-dependent release after neuronal depolarization, and low potency activation of N-methyl-D-aspartate receptors. In the present study, the influence of NAAG on metabotropic receptor activation in cerebellar granule cells was examined in cell culture. Stimulation of granule cell adenylate cyclase with forskolin increased cyclic AMP (cAMP) several hundredfold above basal levels within 10 min in a concentration-dependent manner. Although glutamate, NAAG, and the metabotropic receptor agonist trans-1-amino-1,3-cyclopentanedicarboxylic acid did not alter the low basal cAMP levels, the application of 300 microM glutamate or NAAG or trans-1-amino-1,3-cyclopentanedicarboxylic acid reduced forskolin-stimulated cAMP in granule cells by 30-50% in the absence or presence of inhibitors of ionotropic acidic amino acid receptors, as well as 2-amino-4-phosphonobutyrate. No additivity in the inhibition of cAMP was found when 300 microM NAAG and trans-1-amino-1,3-cyclopentanedicarboxylic acid were coapplied. The beta-analogue of NAAG failed to reduce cAMP levels. Similar effects of NAAG and glutamate were obtained under conditions of inhibition of phosphodiesterase activity and were prevented by pretreatment of the cells with pertussis toxin. These data are consistent with the activation by NAAG of a metabotropic acidic amino acid receptor coupled to an inhibitory G protein. In contrast, the metabotropic acidic amino acid receptor coupled to phosphoinositol turnover in these cells was not activated by NAAG.(ABSTRACT TRUNCATED AT 250 WORDS)

Electrochemical detection of dipeptides with selectivity against amino acids

Electrolysis of a basic mobile phase containing biuret reagent [Cu(II) and a tartrate salt] at high (> 1.2 V vs. Ag/AgCl) potentials modifies the glassy carbon electrode. This modified anode oxidizes dipeptides, yielding signals expected for a one-electron transfer, even at low (down to 0.7 V vs. Ag/AgCl) potentials and in the absence of intentionally added copper(II) ion in the reagent or mobile phase. The same modification demonstrates a selectivity to alpha-dipeptides over amino acids that is unprecedented. The product of the anodic reaction is reduced at a downstream cathode at low positive potentials. Sensitivities for several amino acids and dipeptides are reported under several conditions. Neither the anodic nor the cathodic signals for the buiret complex of the tripeptide Ala-Ala-Ala are significantly altered because of the modification.

N-acetylaspartylglutamate catabolism is achieved by an enzyme on the cell surface of neurons and glia

N-Acetylaspartylglutamate (NAAG) is a nervous system-specific, acidic dipeptide which is released from neurons in a manner consistent with a function in synaptic neurotransmission. The hydrolysis of NAAG to produce glutamate and N-acetylaspartate was analyzed in cell cultures prepared from murine brain cells. Peptidase activity against NAAG was found in cultures which contained both neurons and glia, as well as in cultures of glia alone. Several lines of evidence were obtained in support of the hypothesis that this peptidase activity is predominantly bound to the extracellular face of the plasma membranes of these cells. Glutamate released from NAAG accumulated in the extracellular medium. Extracellular application of peptidase inhibitors effectively reduced NAAG hydrolysis. Peptidase activity was not secreted into the cell culture medium by intact cells, and lysed cells did not release detectable peptidase activity beyond that obtained with intact cells. Replacement of extracellular sodium with choline inhibited peptide uptake while stimulating apparent extracellular NAAG hydrolysis by intact cells in culture. Finally, the steady rise in extracellular glutamate as a consequence of NAAG hydrolysis by these brain cells, including glia, supports the conclusion that glutamate uptake is not tightly coupled to peptidase activity and thus that NAAG serves as a significant source of glutamate in the synaptic space following depolarization-induced peptide release.

Relay cells, not interneurons, of cat's lateral geniculate nucleus contain N-acetylaspartylglutamate

N-acetylaspartylglutamate (NAAG) is an endogenous brain dipeptide that satisfies many of the criteria for a neurotransmitter. We have previously identified NAAG immunoreactivity in neurons of the lateral geniculate nucleus (LGN) of the cat and monkey. To determine whether all LGN neurons contain NAAG, we treated sections of cat LGN with affinity-purified antibodies to NAAG and counterstained them with thionin. The larger neurons contained NAAG, but the smaller neurons did not. We treated other sections with antiserum to glutamic acid decarboxylase (GAD), the rate-limiting enzyme in the synthesis of gamma-aminobutyric acid (GABA), in order to label interneurons of the LGN. In these sections, the smaller cells were labeled; the larger neurons were not. We hypothesized that NAAG was present in relay cells, but not interneurons. We used two double-labeling paradigms to test this hypothesis. We combined immunocytochemistry for NAAG using a fluorescent secondary antibody with either (1) fluorescent retrograde tracers (true blue, granular blue, rhodamine beads, or propidium iodide) injected into areas 17 and/or 18 or (2) immunocytochemistry for GAD using a second fluorescent secondary antibody. In the LGN, over 99% of retrogradely labeled cells contained NAAG, but few GAD-positive neurons did. In contrast, neurons of the perigeniculate nucleus contained both NAAG and GAD, demonstrating that staining by one set of antisera did not inhibit staining by the other and that perigeniculate neurons are chemically distinct from the interneurons of the LGN. We conclude that in LGN, the relay cells, which project to visual cortex, contain NAAG, whereas most of the interneurons, which contain GABA, do not.

Glutamate-like immunoreactivity in retinal terminals of the mouse suprachiasmatic nucleus

With a view to identifying the neurotransmitter content of retinal terminals within the mouse suprachiasmatic nucleus, a highly specific antiserum to glutaraldehyde-coupled glutamate was used in a postembedding immunogold procedure at the ultrastructural level. Retinal terminals were identified by cholera toxin--horseradish peroxidase transported anterogradely from the retina and reacted with tetramethyl benzidine/tungstate/H2O2, or by their characteristically pale and distended mitochondria with irregular cristae. Controls included model ultrathin sections containing high concentrations of various amino acids. Alternate serial sections were labelled with anti-glutamate and anti-gamma-aminobutyric acid (GABA). Data were analysed by computer-assisted image analysis. Density of glutamate labelling (gold particles per micron2) on whole retinal terminals was > 3 times higher than that on postsynaptic dendrites, and > 5 times higher than that on miscellaneous non-retinal non-glutamatergic terminals in the suprachiasmatic nucleus. The overall density of gold particles over retinal terminals was approximately 3 times higher than that over GABAergic terminals, in which glutamate-like immunoreactivity was mainly mitochondrial. Labelling of vesicles in retinal terminals was almost 5 times greater than the apparent labelling of vesicles in GABAergic terminals, underscoring the location of transmitter glutamate within synaptic vesicles in retinal terminals. In the retino-recipient region of the suprachiasmatic nucleus there was also a small population of non-retinal glutamatergic terminals. Their overall immunoreactivity was similar to or exceeded that of retinal terminals, but morphological features clearly distinguished between these two types of glutamate-containing terminals. The present results indicate that the vast majority of retinal terminals may use glutamate as a transmitter, in keeping with electrophysiological and neuropharmacological data from other sources. The possibility of cotransmitters within retinal terminals, suggested by the presence of dense-core vesicles among the glutamate-containing synaptic vesicles, has still to be addressed.

Uptake, Metabolism, and Release of N-[3H]-Acetylaspartylglutamate by the Avian Retina

N-Acetylaspartylglutamate (NAAG) is a nervous system-specific dipeptide that is released from retinal neurons on depolarization. In the present study, extracellular metabolism, uptake, and release of [3H]NAAG were examined in the chick retina. After in vitro incubation with NAAG radiolabeled in the glutamate moiety, [3H]glutamate and [3H]NAAG increased in retinal cells through time- and temperature-dependent processes, which were reduced in the absence of extracellular sodium. Coincubation of cells with [3H]NAAG and aspartylglutamate or phosphate resulted in the decreased extracellular appearance of [3H]glutamate, produced by hydrolysis of radiolabeled NAAG, and a consequent increased availability of [3H]NAAG for transport into the retinal cells. When this tissue was incubated with radiolabeled NAAG, glutamate, glutamine, or aspartate under similar conditions, only [3H]NAAG served as a significant source for the appearance of intracellular [3H]NAAG. These data support the conclusion that [3H]NAAG can be transported into retinal cells, whereas [3H]glutamate transport is the predominant process after release of this amino acid from NAAG by extracellular peptidase activities. After uptake, [3H]NAAG entered a cellular pool, from which the peptide was secreted under depolarizing conditions and in a calcium-dependent manner.

N-acetylaspartylglutamate immunoreactivity in neurons of the monkey's visual pathway

The acidic dipeptide N-acetylaspartylglutamate (NAAG) was identified immunohistochemically within neurons of the visual pathways of two adult macaque monkeys which had undergone midsagittal sectioning of the optic chiasm 6 or 9 years earlier. In both temporal and nasal retinae, amacrine cells, including some displaced amacrine cells, expressed NAAG immunoreactivity. In temporal but not nasal retina, retinal ganglion cells were stained, as were their dendrites in the inner plexiform layer, and their axons in the optic nerve fiber layer. In nasal retina, the ganglion cells had degenerated because they were axotomized by the optic chiasm section. In the target regions of the retinal ganglion cells, the superior colliculus and the lateral geniculate nucleus (LGN), both neuropil and cell bodies were stained. In LGN, staining was confined to layers 2, 3, and 5, that is, to the layers innervated by the intact ipsilateral pathway. Immunoreactivity was also seen in the cells of layers 2, 3A, 4B, 5, and 6 of area 17 and layers 3 and 5 of area 18. The neuropil was stained in all layers of area 17, but more heavily in layers 1, 2, 4B, the bottom of 4C beta, 5B, and 6B. Within 4C the staining was patchy; in tangential sections there were alternating bands of light and dark label which matched the ocular dominance bands demonstrated by cytochrome oxidase histochemistry in adjacent sections. This banding pattern is consistent with the presence of NAAG in geniculocortical terminals of the intact ipsilateral pathway and the absence of such terminals for the contralateral pathway, which had undergone transneuronal degeneration due to the optic chiasm sectioning. Overall, our results for monkey are very similar to those in cat and suggest that NAAG or a structurally related molecule may have a prominent role in the communication of visual signals at retinal, thalamic, and cortical levels.

Effect of eye removal on N-acetylaspartylglutamate immunoreactivity in retinal targets of the cat

The endogenous brain dipeptide N-acetylaspartylglutamate (NAAG) has previously been demonstrated in the somata of retinal ganglion cells and the neuropil of retinal targets. In this paper we report that the NAAG immunoreactivity of the neuropil in the retinal targets is dependent on an intact optic pathway. Removal of one eye produced a marked decrease in the staining of the neuropil in layer A of the contralateral geniculate nucleus (LGN) and layer A1 of the ipsilateral LGN. There was also decreased staining in the superficial layers of the superior colliculus contralateral to the removal. These results suggest that NAAG is present in the terminals of retinal ganglion cells and is consistent with a role for NAAG in visual synaptic transmission.