The neuroactive peptide N-acetylaspartylglutamate is not an agonist at the metabotropic glutamate receptor subtype 3 of metabotropic glutamate receptor

An enzymatic fluorimetric assay for determination of N-acetylaspartate

N-acetylaspartate (NAA) is an abundant metabolite in the mammalian brain and a precursor of the neuropeptide N-acetylaspartylglutamate (NAAG). The physiological role of NAA is not fully understood and requires further studies. We here describe the development of a coupled enzymatic fluorimetric assay for the determination of NAA in biological samples. Deproteinized tissue extracts are first passed through a strong cation exchange column to remove aspartate. NAA in the sample is hydrolysed by aspartoacylase and released aspartate oxidized using l-aspartate oxidase. Generated H₂O₂ is measured with peroxidase in a fluorimetric assay using Ampliflu Red. The limit of detection and the lower limit of quantification are 1.0 μM (10 pmol/well) and 3.3 μM (33 pmol/well), respectively, with a linear range to 100 μM. Specificity of the assay was confirmed using samples from mice deficient in NAA synthase Nat8l that were spiked with NAA. Analysis of samples from aspartoacylase-deficient mice showed a 2 to 3-fold increase in brain NAA concentration, in line with previous reports. Mice lacking NAAG synthetases had a slightly reduced (-10%) brain NAA level. Thus, the new fluorimetric enzymatic assay is useful to perform sensitive and large scale quantification of NAA in biological samples without the need for expensive equipment.

The role of thalamic group II mGlu receptors in health and disease

The thalamus plays a pivotal role in the integration and processing of sensory, motor, and cognitive information. It is therefore important to understand how the thalamus operates in states of both health and disease. In the present review, we discuss the function of the Group II metabotropic glutamate (mGlu) receptors within thalamic circuitry, and how they may represent therapeutic targets in treating disease states associated with thalamic dysfunction.

N-Acetylaspartyl-Glutamate Metabolism in the Cingulated Cortices as a Biomarker of the Etiology in ASD: A 1H-MRS Model

As brain functional resonance magnetic studies show an aberrant trajectory of neurodevelopment, it is reasonable to predict that the degree of neurochemical abnormalities indexed by magnetic resonance spectroscopy (1H-MRS) might also change according to the developmental stages and brain regions in autism spectrum disorders (ASDs). Since specific N-Acetyl-aspartate (NAA) changes in children's metabolism have been found in the anterior cingulate cortex (ACC) but not in the posterior cingulate cortex (PCC), we analyzed whether the metabolites of ASD youths change between the cingulate cortices using 1H-MRS. l-glutamate (Glu) and l-Acetyl-aspartate (NAA) are products from the N-Acetyl-aspartyl-glutamate (NAAG) metabolism in a reaction that requires the participation of neurons, oligodendrocytes, and astrocytes. This altered tri-cellular metabolism has been described in several neurological diseases, but not in ASD. Compared to the typical development (TD) group, the ASD group had an abnormal pattern of metabolites in the ACC, with a significant increase of glutamate (12.10 ± 3.92 mM; p = 0.02); additionally, N-Acetyl-aspartyl-glutamate significantly decreased (0.41 ± 0.27 mM; p = 0.02) within ASD metabolism abnormalities in the ACC, which may allow the development of new therapeutic possibilities.

The activation of group II metabotropic glutamate receptors protects neonatal rat brains from oxidative stress injury after hypoxia-ischemia

Birth asphyxia resulting in brain hypoxia-ischemia (H-I) can cause neonatal death or lead to persistent brain damage. Recent investigations have shown that group II metabotropic glutamate receptor (mGluR2/3) activation can provide neuroprotection against H-I but the mechanism of this effect is not clear. The aim of this study was to investigate whether mGluR2/3 agonists applied a short time after H-I reduce brain damage in an experimental model of birth asphyxia, and whether a decrease in oxidative stress plays a role in neuroprotection. Neonatal H-I in 7-day-old rats was used as an experimental model of birth asphyxia. Rats were injected intra peritoneally with mGluR2 (LY 379268) or mGluR3 (NAAG) agonists 1 h or 6 h after H-I (5 mg/kg). The weight deficit of the ischemic brain hemisphere, radical oxygen species (ROS) content levels, antioxidant enzymes activity and the concentrations of reduced glutathione (GSH) were measured. Both agonists reduced weight loss in the ischemic hemisphere and mitigated neuronal degeneration in the CA1 hippocampal region and cerebral cortex. Both agonists reduced the elevated levels of ROS in the ipsilateral hemisphere observed after H-I and prevented an increase in antioxidant enzymes activity in the injured hemisphere restoring them to control levels. A decrease in GSH level was also restored after agonists application. The results show that the activation of mGluR2 and mGluR3 a short time after H-I triggers neuroprotective mechanisms that act through the inhibition of oxidative stress and ROS production. The prevention of ROS production by the inhibition of glutamate release and decrease in its extracellular concentration is likely the main mechanism involved in the observed neuroprotection.

Synthesis and Pharmacological Characterization of C4β-Amide-Substituted 2-Aminobicyclo[3.1.0]hexane-2,6-dicarboxylates. Identification of (1 S,2 S,4 S,5 R,6 S)-2-Amino-4-[(3-methoxybenzoyl)amino]bicyclo[3.1.0]hexane-2,6-dicarboxylic Acid (LY2794193), a Highly Potent and Selective mGlu3 Receptor Agonist

Multiple therapeutic opportunities have been suggested for compounds capable of selective activation of metabotropic glutamate 3 (mGlu₃) receptors, but small molecule tools are lacking. As part of our ongoing efforts to identify potent, selective, and systemically bioavailable agonists for mGlu₂ and mGlu₃ receptor subtypes, a series of C4_β-N-linked variants of (1 S,2 S,5 R,6 S)-2-amino-bicyclo[3.1.0]hexane-2,6-dicarboxylic acid 1 (LY354740) were prepared and evaluated for both mGlu₂ and mGlu₃ receptor binding affinity and functional cellular responses. From this investigation we identified (1 S,2 S,4 S,5 R,6 S)-2-amino-4-[(3-methoxybenzoyl)amino]bicyclo[3.1.0]hexane-2,6-dicarboxylic acid 8p (LY2794193), a molecule that demonstrates remarkable mGlu₃ receptor selectivity. Crystallization of 8p with the amino terminal domain of hmGlu₃ revealed critical binding interactions for this ligand with residues adjacent to the glutamate binding site, while pharmacokinetic assessment of 8p combined with its effect in an mGlu₂ receptor-dependent behavioral model provides estimates for doses of this compound that would be expected to selectively engage and activate central mGlu₃ receptors in vivo.

Striatal N-Acetylaspartate Synthetase Shati/Nat8l Regulates Depression-Like Behaviors via mGluR3-Mediated Serotonergic Suppression in Mice

BACKGROUND

Several clinical studies have suggested that N-acetylaspartate and N-acetylaspartylglutamate levels in the human brain are associated with various psychiatric disorders, including major depressive disorder. We have previously identified Shati/Nat8l, an N-acetyltransferase, in the brain using an animal model of psychosis. Shati/Nat8l synthesizes N-acetylaspartate from L-aspartate and acetyl-coenzyme A. Further, N-acetylaspartate is converted into N-acetylaspartylglutamate, a neurotransmitter for metabotropic glutamate receptor 3.

METHODS

Because Shati/Nat8l mRNA levels were increased in the dorsal striatum of mice following the exposure to forced swimming stress, Shati/Nat8l was overexpressed in mice by the microinjection of adeno-associated virus vectors containing Shati/Nat8l gene into the dorsal striatum (dS-Shati/Nat8l mice). The dS-Shati/Nat8l mice were further assessed using behavioral and neurochemical tests.

RESULTS

The dS-Shati/Nat8l mice exhibited behavioral despair in the forced swimming and tail suspension tests and social withdrawal in the 3-chamber social interaction test. These depression-like behaviors were attenuated by the administration of a metabotropic glutamate receptor 2/3 antagonist and a selective serotonin reuptake inhibitor. Furthermore, the metabolism of N-acetylaspartate to N-acetylaspartylglutamate was decreased in the dorsal striatum of the dS-Shati/Nat8l mice. This finding corresponded with the increased expression of glutamate carboxypeptidase II, an enzyme that metabolizes N-acetylaspartylglutamate present in the extracellular space. Extracellular serotonin levels were lower in the dorsal striatum of the dS-Shati/Nat8l and normal mice that were repeatedly administered a selective glutamate carboxypeptidase II inhibitor.

CONCLUSIONS

Our findings indicate that the striatal expression of N-acetylaspartate synthetase Shati/Nat8l plays a role in major depressive disorder via the metabotropic glutamate receptor 3-mediated functional control of the serotonergic neuronal system.

NAAG Peptidase Inhibitors Act via mGluR3: Animal Models of Memory, Alzheimer's, and Ethanol Intoxication

Glutamate carboxypeptidase II (GCPII) inactivates the peptide neurotransmitter N-acetylaspartylglutamate (NAAG) following synaptic release. Inhibitors of GCPII increase extracellular NAAG levels and are efficacious in animal models of clinical disorders via NAAG activation of a group II metabotropic glutamate receptor. mGluR2 and mGluR3 knock-out (ko) mice were used to test the hypothesis that mGluR3 mediates the activity of GCPII inhibitors ZJ43 and 2-PMPA in animal models of memory and memory loss. Short- (1.5 h) and long- (24 h) term novel object recognition tests were used to assess memory. Treatment with ZJ43 or 2-PMPA prior to acquisition trials increased long-term memory in mGluR2, but not mGluR3, ko mice. Nine month-old triple transgenic Alzheimer's disease model mice exhibited impaired short-term novel object recognition memory that was rescued by treatment with a NAAG peptidase inhibitor. NAAG peptidase inhibitors and the group II mGluR agonist, LY354740, reversed the short-term memory deficit induced by acute ethanol administration in wild type mice. 2-PMPA also moderated the effect of ethanol on short-term memory in mGluR2 ko mice but failed to do so in mGluR3 ko mice. LY354740 and ZJ43 blocked ethanol-induced motor activation. Both GCPII inhibitors and LY354740 also significantly moderated the loss of motor coordination induced by 2.1 g/kg ethanol treatment. These data support the conclusion that inhibitors of glutamate carboxypeptidase II are efficacious in object recognition models of normal memory and memory deficits via an mGluR3 mediated process, actions that could have widespread clinical applications.

The Good and Bad Sides of NAAG

Why has such a small peptide been the source of controversy in neuroscience over the last 5 decades? Is N-acetyl-aspartyl-glutamate (NAAG) a neurotransmitter? Is NAAG located in neuronal tissue or in astrocytes? Is NAAG involved in neuropsychiatric and neurodegenerative disorders? Is NAAG therapeutically beneficial in the treatment of stroke or in initiating cascades of events leading to psychosis? After many years of intense research there is no clear consensus within the scientific community on how NAAG behaves in the brain. One of the major controversies about NAAG is its physiological action at N-methyl-d-aspartate (NMDA) receptors. While some researchers strongly argue that NAAG acts as a weak agonist at NMDA receptors, others have suggested that NAAG could behave as a potent antagonist. Published data from our laboratory demonstrate that the effect of NAAG on NMDA receptors could be influenced by a number of factors including the subcellular localization and subunit composition of NMDA receptors, as well as protons. In this chapter, we will summarize the knowledge of the literature on NAAG, however, we will place emphasis on our recently published data. More specifically, we have reported interesting findings on the effects of NAAG on NMDA receptors at synaptic and extrasynaptic sites using a pharmacological paradigm to distinguish the two populations of NMDA receptors. Additionally, we have evaluated the role of NAAG on GluN2A- and GluN2B-containing NMDA receptors using a HEK293 cell recombinant system. Finally, we have studied the effects of NAAG on GluN2A- and GluN2B-containing NMDA receptors in different extracellular pH conditions. We believe that our findings could potentially resolve some aspects of the debate regarding the role of NAAG at NMDA receptors.

Selective CNS Uptake of the GCP-II Inhibitor 2-PMPA following Intranasal Administration

Glutamate carboxypeptidase II (GCP-II) is a brain metallopeptidase that hydrolyzes the abundant neuropeptide N-acetyl-aspartyl-glutamate (NAAG) to NAA and glutamate. Small molecule GCP-II inhibitors increase brain NAAG, which activates mGluR3, decreases glutamate, and provide therapeutic utility in a variety of preclinical models of neurodegenerative diseases wherein excess glutamate is presumed pathogenic. Unfortunately no GCP-II inhibitor has advanced clinically, largely due to their highly polar nature resulting in insufficient oral bioavailability and limited brain penetration. Herein we report a non-invasive route for delivery of GCP-II inhibitors to the brain via intranasal (i.n.) administration. Three structurally distinct classes of GCP-II inhibitors were evaluated including DCMC (urea-based), 2-MPPA (thiol-based) and 2-PMPA (phosphonate-based). While all showed some brain penetration following i.n. administration, 2-PMPA exhibited the highest levels and was chosen for further evaluation. Compared to intraperitoneal (i.p.) administration, equivalent doses of i.n. administered 2-PMPA resulted in similar plasma exposures (AUC_{0-t, i.n}./AUC_{0-t, i.p.} = 1.0) but dramatically enhanced brain exposures in the olfactory bulb (AUC_{0-t, i.n}./AUC_{0-t, i.p.} = 67), cortex (AUC_{0-t, i.n}./AUC_{0-t, i.p.} = 46) and cerebellum (AUC_{0-t, i.n}./AUC_{0-t, i.p.} = 6.3). Following i.n. administration, the brain tissue to plasma ratio based on AUC_0-t in the olfactory bulb, cortex, and cerebellum were 1.49, 0.71 and 0.10, respectively, compared to an i.p. brain tissue to plasma ratio of less than 0.02 in all areas. Furthermore, i.n. administration of 2-PMPA resulted in complete inhibition of brain GCP-II enzymatic activity ex-vivo confirming target engagement. Lastly, because the rodent nasal system is not similar to humans, we evaluated i.n. 2-PMPA also in a non-human primate. We report that i.n. 2-PMPA provides selective brain delivery with micromolar concentrations. These studies support intranasal delivery of 2-PMPA to deliver therapeutic concentrations in the brain and may facilitate its clinical development.

The Concise Guide to PHARMACOLOGY 2013/14: G protein-coupled receptors

The Concise Guide to PHARMACOLOGY 2013/14 provides concise overviews of the key properties of over 2000 human drug targets with their pharmacology, plus links to an open access knowledgebase of drug targets and their ligands (www.guidetopharmacology.org), which provides more detailed views of target and ligand properties. The full contents can be found at http://onlinelibrary.wiley.com/doi/10.1111/bph.12444/full. G protein-coupled receptors are one of the seven major pharmacological targets into which the Guide is divided, with the others being G protein-coupled receptors, ligand-gated ion channels, ion channels, catalytic receptors, nuclear hormone receptors, transporters and enzymes. These are presented with nomenclature guidance and summary information on the best available pharmacological tools, alongside key references and suggestions for further reading. A new landscape format has easy to use tables comparing related targets. It is a condensed version of material contemporary to late 2013, which is presented in greater detail and constantly updated on the website www.guidetopharmacology.org, superseding data presented in previous Guides to Receptors and Channels. It is produced in conjunction with NC-IUPHAR and provides the official IUPHAR classification and nomenclature for human drug targets, where appropriate. It consolidates information previously curated and displayed separately in IUPHAR-DB and the Guide to Receptors and Channels, providing a permanent, citable, point-in-time record that will survive database updates.

Long-term follow-up after gene therapy for canavan disease

Canavan disease is a hereditary leukodystrophy caused by mutations in the aspartoacylase gene (ASPA), leading to loss of enzyme activity and increased concentrations of the substrate N-acetyl-aspartate (NAA) in the brain. Accumulation of NAA results in spongiform degeneration of white matter and severe impairment of psychomotor development. The goal of this prospective cohort study was to assess long-term safety and preliminary efficacy measures after gene therapy with an adeno-associated viral vector carrying the ASPA gene (AAV2-ASPA). Using noninvasive magnetic resonance imaging and standardized clinical rating scales, we observed Canavan disease in 28 patients, with a subset of 13 patients being treated with AAV2-ASPA. Each patient received 9 × 10(11) vector genomes via intraparenchymal delivery at six brain infusion sites. Safety data collected over a minimum 5-year follow-up period showed a lack of long-term adverse events related to the AAV2 vector. Posttreatment effects were analyzed using a generalized linear mixed model, which showed changes in predefined surrogate markers of disease progression and clinical assessment subscores. AAV2-ASPA gene therapy resulted in a decrease in elevated NAA in the brain and slowed progression of brain atrophy, with some improvement in seizure frequency and with stabilization of overall clinical status.

Glycine release is regulated by metabotropic glutamate receptors sensitive to mGluR2/3 ligands and activated by N-acetylaspartylglutamate (NAAG)

The presence of metabotropic glutamate receptors (mGluRs) of group II modulating glycine exocytosis from glycinergic nerve endings of mouse spinal cord was investigated. Purified synaptosomes were selectively prelabeled with [(3)H]glycine through the neuronal transporter GlyT2 and subsequently depolarized by superfusion with 12 mM KCl. The selective mGluR2/3 agonist LY379268 inhibited the K(+)-evoked overflow of [(3)H]glycine in a concentration-dependent manner (EC(50) about 0.2 nM). The effect of LY379268 was prevented by the selective mGluR2/3 antagonist LY341495 (IC(50) about 1 nM). N-acetylaspartylglutamate (NAAG) inhibited [(3)H]glycine overflow with extraordinary potency (EC(50) about 50 fmol). In contrast, glutamate was ineffective up to 0.1 nM, excluding that glutamate contamination of commercial NAAG samples is responsible for the reported activity of NAAG at mGluR3. LY341495 antagonized the NAAG inhibition of [(3)H]glycine release. The effect of a combination of maximally effective concentrations of LY379268 and NAAG exhibited no additivity. The non-hydrolysable NAAG analogue N-acetylaspartyl-β-linked glutamate (β-NAAG) antagonized NAAG and LY379268. In conclusion, our results show that glycinergic nerve endings in spinal cord are endowed with group II mGluRs mediating inhibition of glycine exocytosis. NAAG can activate these presynaptic receptors with extremely high affinity and with characteristics compatible with the reported mGluR3 pharmacology. This article is part of a Special Issue entitled 'Metabotropic Glutamate Receptors'.

Metabotropic glutamate receptors as targets for analgesia: antagonism, activation, and allosteric modulation

The metabotropic glutamate receptors (mGluRs) are expressed pre- and post-synaptically throughout the nervous system where they serve as modulators of synaptic transmission and neuronal excitability. Activation of mGluRs can be pro- or anti-nociceptive, depending on their anatomic location and the signaling cascades to which they couple. Antagonists of Group I mGluRs and agonists of Group II and III mGluRs have shown therapeutic promise in animal pain models. This article reviews the potential therapeutic utility of several agents that act predominantly via mGluRs, specifically focusing on their analgesic efficacy and discussing possible off-target effects. Glutamate, the primary excitatory neurotransmitter in the vertebrate nervous system, mediates its effects via activation of two main classes of receptors: ligand-gated ion channels known as ionotropic receptors and G-protein coupled metabotropic receptors. Antagonists of ionotropic glutamate receptors, such as ketamine, have robust analgesic properties; however, their analgesic utility is limited to monitored clinical settings due to the potential for psychomimetic effects.

mGlu3 receptor and astrocytes: partners in neuroprotection

Astrocytes are currently studied intensively because of their now highlighted relevance as key players with neurons that modulate a wide range of central functions, from synaptic plasticity and synaptogenesis to regulation of metabolic and neuroinflammatory processes. Since the discovery of mGlu3 receptors on astrocytes, accumulating evidence supports a role of these receptors not only in maintaining synaptic homeostasis and treating psychiatric disorders but also in promoting astrocyte survival in several pathologic conditions. This review focuses on providing up-to-date knowledge regarding effects of activating astroglial mGlu3 receptors on psychiatric disorders, astrocyte and neuronal survival, and neurodegenerative diseases. This article is part of a Special Issue entitled 'Metabotropic Glutamate Receptors'.

Selective activation of either mGlu2 or mGlu3 receptors can induce LTD in the amygdala

Group II metabotropic glutamate (mGlu) receptors are known to induce a long-term depression (LTD) of synaptic transmission in many brain regions including the amygdala. However the roles of the individual receptor subtypes, mGlu2 and mGlu3, in LTD are not well understood. In particular, it is unclear whether activation of mGlu3 receptors is sufficient to induce LTD at synapses in the CNS. In the present study, advantage was taken of a Wistar rat strain not expressing mGlu2 receptors (Ceolin et al., 2011) to investigate the function of mGlu3 receptors in the amygdala. In this preparation, the group II agonist, DCG-IV induced an LTD of the cortical, but not the intra-nuclear, synaptic input to the lateral amygdala. This LTD was concentration dependent and was blocked by the group II mGlu receptor antagonist, LY341495. To investigate further the role of mGlu3 receptors, we used LY395756 (an mGlu2 agonist and mGlu3 antagonist), which acts as a pure mGlu3 receptor antagonist in this rat strain. This compound alone had no effect on basal synaptic transmission, but blocked the LTD induced by DCG-IV. Furthermore, we found that DCG-IV also induces LTD in mGlu2 receptor knock-out (KO) mice to a similar extent as in wild-type mice. This confirms that the activation of mGlu3 receptors alone is sufficient to induce LTD at this amygdala synapse. To address whether mGlu2 activation alone is also sufficient to induce LTD at this synapse we used LY541850 (the active enantiomer of LY395756) in wild-type mice. LY541850 induced a substantial LTD showing that either receptor alone is capable of inducing LTD in this pathway. This article is part of a Special Issue entitled 'Metabotropic Glutamate Receptors'.

MR spectroscopic studies of the brain in psychiatric disorders

The measurement of brain metabolites with magnetic resonance spectroscopy (MRS) provides a unique perspective on the brain bases of neuropsychiatric disorders. As a context for interpreting MRS studies of neuropsychiatric disorders, we review the characteristic MRS signals, the metabolic dynamics,and the neurobiological significance of the major brain metabolites that can be measured using clinical MRS systems. These metabolites include N-acetylaspartate(NAA), creatine, choline-containing compounds, myo-inositol, glutamate and glutamine, lactate, and gamma-amino butyric acid (GABA). For the major adult neuropsychiatric disorders (schizophrenia, bipolar disorder, major depression, and the anxiety disorders), we highlight the most consistent MRS findings, with an emphasis on those with potential clinical or translational significance. Reduced NAA in specific brain regions in schizophrenia, bipolar disorder, post-traumatic stress disorder, and obsessive–compulsive disorder corroborate findings of reduced brain volumes in the same regions. Future MRS studies may help determine the extent to which the neuronal dysfunction suggested by these findings is reversible in these disorders. Elevated glutamate and glutamine (Glx) in patients with bipolar disorder and reduced Glx in patients with unipolar major depression support models of increased and decreased glutamatergic function, respectively, in those conditions. Reduced phosphomonoesters and intracellular pH in bipolar disorder and elevated dynamic lactate responses in panic disorder are consistent with metabolic models of pathogenesis in those disorders. Preliminary findings of an increased glutamine/glutamate ratio and decreased GABA in patients with schizophrenia are consistent with a model of NMDA hypofunction in that disorder. As MRS methods continue to improve, future studies may further advance our understanding of the natural history of psychiatric illnesses, improve our ability to test translational models of pathogenesis, clarify therapeutic mechanisms of action,and allow clinical monitoring of the effects of interventions on brain metabolicmarkers

Molecular identification of β-citrylglutamate hydrolase as glutamate carboxypeptidase 3

β-Citrylglutamate (BCG), a compound present in adult testis and in the CNS during the pre- and perinatal periods is synthesized by an intracellular enzyme encoded by the RIMKLB gene and hydrolyzed by an as yet unidentified ectoenzyme. To identify β-citrylglutamate hydrolase, this enzyme was partially purified from mouse testis and characterized. Interestingly, in the presence of Ca(2+), the purified enzyme specifically hydrolyzed β-citrylglutamate and did not act on N-acetyl-aspartylglutamate (NAAG). However, both compounds were hydrolyzed in the presence of Mn(2+). This behavior and the fact that the enzyme was glycosylated and membrane-bound suggested that β-citrylglutamate hydrolase belonged to the same family of protein as glutamate carboxypeptidase 2 (GCP2), the enzyme that catalyzes the hydrolysis of N-acetyl-aspartylglutamate. The mouse tissue distribution of β-citrylglutamate hydrolase was strikingly similar to that of the glutamate carboxypeptidase 3 (GCP3) mRNA, but not that of the GCP2 mRNA. Furthermore, similarly to β-citrylglutamate hydrolase purified from testis, recombinant GCP3 specifically hydrolyzed β-citrylglutamate in the presence of Ca(2+), and acted on both N-acetyl-aspartylglutamate and β-citrylglutamate in the presence of Mn(2+), whereas recombinant GCP2 only hydrolyzed N-acetyl-aspartylglutamate and this, in a metal-independent manner. A comparison of the structures of the catalytic sites of GCP2 and GCP3, as well as mutagenesis experiments revealed that a single amino acid substitution (Asn-519 in GCP2, Ser-509 in GCP3) is largely responsible for GCP3 being able to hydrolyze β-citrylglutamate. Based on the crystal structure of GCP3 and kinetic analysis, we propose that GCP3 forms a labile catalytic Zn-Ca cluster that is critical for its β-citrylglutamate hydrolase activity.

Group II mGluR agonist LY354740 and NAAG peptidase inhibitor effects on prepulse inhibition in PCP and D-amphetamine models of schizophrenia

RATIONALE

Group II metabotropic glutamate receptor (mGluR) agonists represent a novel approach to the treatment of schizophrenia. Inasmuch as the peptide neurotransmitter N-acetylaspartylglutamate (NAAG) activates these receptors, NAAG peptidase inhibitors conceptually represent a parallel path toward development of new antipsychotic drugs. While group II agonists are effective in several animal models of schizophrenia, they are reported to lack efficacy in moderating the effects of phencyclidine (PCP) on prepulse inhibition of acoustic startle in animal models of sensory processing deficits found in this disorder.

OBJECTIVE

The objective of this study was to re-examine the efficacy of a group II metabotropic glutamate agonist and NAAG peptidase inhibitors in prepulse inhibition models of schizophrenia across two strains of mice.

METHODS

The method used was an assay to determine the efficacy of these drugs in moderating the reduction in prepulse inhibition of acoustic startle in mice treated with PCP and D: -amphetamine.

RESULTS

The group II agonist LY354740 (5 and 10 mg/kg) moderated the effects of PCP on prepulse inhibition of acoustic startle in DBA/2 but not C57BL/6 mice. In contrast, two NAAG peptidase inhibitors, ZJ43 (150 mg/kg) and 2-PMPA (50, 100, and 150 mg/kg), did not significantly affect the PCP-induced reduction in prepulse inhibition in either strain.

CONCLUSIONS

These data demonstrate that the efficacy of group II agonists in this model of sensory motor processing is strain-specific in mice. The difference between the effects of the group II agonist and the peptidase inhibitors in the DBA/2 mice may relate to the difference in efficacy of NAAG and the agonist at mGluR2.

Advances in understanding the peptide neurotransmitter NAAG and appearance of a new member of the NAAG neuropeptide family

A substantial body of data was reported between 1984 and 2000 demonstrating that the neuropeptide N-acetylaspartylglutamate (NAAG) not only functions as a neurotransmitter but also is the third most prevalent transmitter in the mammalian nervous system behind glutamate and GABA. By 2005, this conclusion was validated further through a series of studies in vivo and in vitro. The primary enzyme responsible for the inactivation of NAAG following its synaptic release had been cloned, characterized and knocked out. Potent inhibitors of this enzyme were developed and their efficacy has been extensively studied in a series of animal models of clinical conditions, including stroke, peripheral neuropathy, traumatic brain injury, inflammatory and neuropathic pain, cocaine addiction, and schizophrenia. Considerable progress also has been made in defining further the mechanism of action of these peptidase inhibitors in elevating synaptic levels of NAAG with the consequent inhibition of transmitter release via the activation of pre-synaptic metabotropic glutamate receptor 3 by this peptide. Very recent discoveries include identification of two different nervous system enzymes that mediate the synthesis of NAAG from N-acetylaspartate and glutamate and the finding that one of these enzymes also mediates the synthesis of a second member of the NAAG family of neuropeptides, N-acetylaspartylglutamylglutamate.

mGlu2 metabotropic glutamate receptors restrain inflammatory pain and mediate the analgesic activity of dual mGlu2/mGlu3 receptor agonists

Group II metabotropic glutamate receptors (mGluRs) couple to the inhibitory G-protein Gi. The group II mGluRs include two subtypes, mGlu2 and mGlu3, and their pharmacological activation produces analgesic effects in inflammatory and neuropathic pain states. However, the specific contribution of each one of the two subtypes has not been clarified due to the lack of selective orthosteric ligands that can discriminate between mGlu2 and mGlu3 subtypes.

In this study we used mGlu2 or mGlu3 knock-out mice to dissect the specific role for these two receptors in the endogenous control of inflammatory pain and their specific contribution to the analgesic activity of mixed mGlu2/3 receptor agonists.

Our results showed that mGlu2^-/- mice display a significantly greater pain response compared to their wild type littermates. Interestingly the increased pain sensitivity in mGlu2^-/- mice occurred only in the second phase of the formalin test. No differences were observed in the first phase. In contrast, mGlu3^-/- mice did not significantly differ from their wild type littermates in either phase of the formalin test.

When systemically injected, a single administration of the mGlu2/3 agonist, LY379268 (3 mg/kg, ip), showed a significant reduction of both phases in wild-type mice and in mGlu3^-/- but not in mGlu2^-/- mice. However tolerance to the analgesic effect of LY379268 (3 mg/kg, ip) in mGlu3^-/- mice developed following 5 consecutive days of injection.

Taken together, these results demonstrate that: (i) mGlu2 receptors play a predominant role over mGlu3 receptors in the control of inflammatory pain in mice; (ii) the analgesic activity of mixed mGlu2/3 agonists is entirely mediated by the activation of the mGlu2 subtype and (iii) the development of tolerance to the analgesic effect of mGlu2/3 agonists develops despite the lack of mGlu3 receptors.

Type 2 metabotropic glutamate receptor (mGluR2) fails to negatively couple to cGMP in stably transfected cells

The group II metabotropic glutamate receptors 2 and 3 (mGluR2 and mGluR3) share sequence homology, common pharmacology and negative coupling to cAMP. We recently discovered that mGluR3 also is negatively coupled through a G-protein to the cGMP transduction pathway in rat cerebellar granule cells and astrocytes. To test the hypothesis that mGluR2 also has access to the cGMP pathway, C6 glioma cells were stably transfected with mGluR2 and mGluR3 cDNA and their coupling to cGMP levels was characterized. In contrast to many other cell lines, C6 has a robust cGMP response that makes it attractive in the study of receptor coupling to this second messenger pathway. Consistent with prior studies, the mGluR3 receptor was negatively coupled to cGMP and this coupling was blocked by PTX. In contrast, mGluR2 agonists failed to reduce sodium nitroprusside stimulated cGMP levels in transfected cell lines where the receptor was negatively coupled to cAMP. These data provide further support for the functional divergence between these two closely related receptors.

Metabotropic glutamate receptors: from the workbench to the bedside

Metabotropic glutamate (mGlu) receptors were discovered in the mid 1980s and originally described as glutamate receptors coupled to polyphosphoinositide hydrolysis. Almost 6500 articles have been published since then, and subtype-selective mGlu receptor ligands are now under clinical development for the treatment of a variety of disorders such as Fragile-X syndrome, schizophrenia, Parkinson's disease and L-DOPA-induced dyskinesias, generalized anxiety disorder, chronic pain, and gastroesophageal reflux disorder. Prof. Erminio Costa was linked to the early times of the mGlu receptor history, when a few research groups challenged the general belief that glutamate could only activate ionotropic receptors and all metabolic responses to glutamate were secondary to calcium entry. This review moves from those nostalgic times to the most recent advances in the physiology and pharmacology of mGlu receptors, and highlights the role of individual mGlu receptor subtypes in the pathophysiology of human disorders. This article is part of a Special Issue entitled 'Trends in neuropharmacology: in memory of Erminio Costa'.

Molecular identification of N-acetylaspartylglutamate synthase and beta-citrylglutamate synthase

The purpose of the present work was to determine the identity of the enzymes that synthesize N-acetylaspartylglutamate (NAAG), the most abundant dipeptide present in vertebrate central nervous system (CNS), and β-citrylglutamate, a structural analogue of NAAG present in testis and immature brain. Previous evidence suggests that NAAG is not synthesized on ribosomes but presumably is synthesized by a ligase. As attempts to detect this ligase in brain extracts failed, we searched the mammalian genomes for putative enzymes that could catalyze this type of reaction. Mammalian genomes were found to encode two putative ligases homologous to Escherichia coli RIMK, which ligates glutamates to the C terminus of ribosomal protein S6. One of them, named RIMKLA, is almost exclusively expressed in the CNS, whereas RIMKLB, which shares 65% sequence identity with RIMKLA, is expressed in CNS and testis. Both proteins were expressed in bacteria or HEK293T cells and purified. RIMKLA catalyzed the ATP-dependent synthesis of N-acetylaspartylglutamate from N-acetylaspartate and l-glutamate. RIMKLB catalyzed this reaction as well as the synthesis of β-citrylglutamate. The nature of the reaction products was confirmed by mass spectrometry and NMR. RIMKLA was shown to produce stoichiometric amounts of NAAG and ADP, in agreement with its belonging to the ATP-grasp family of ligases. The molecular identification of these two enzymes will facilitate progress in the understanding of the function of NAAG and β-citrylglutamate.

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.

Inhibition of NAALADase by 2-PMPA attenuates cocaine-induced relapse in rats: a NAAG-mGluR2/3-mediated mechanism

Pharmacological activation of group II metabotropic glutamate receptors (mGluR2/3) inhibits cocaine self-administration and reinstatement of drug-seeking behavior, suggesting a possible use of mGluR2/3 agonists in the treatment of cocaine dependence. In this study, we investigated whether elevation of the endogenous mGluR2/3 ligand N-acetyl-aspartatylglutamate (NAAG) levels by the N-acetylated-alpha-linked-acidic dipeptidase inhibitor 2-(phosphonomethyl)pentanedioic acid (2-PMPA) attenuates cocaine self-administration and cocaine-induced reinstatement of drug seeking. N-acetylated-alpha-linked-acidic dipeptidase is a NAAG degradation enzyme that hydrolyzes NAAG to N-acetylaspartate and glutamate. Systemic administration of 2-PMPA (10-100 mg/kg, i.p.) inhibited intravenous self-administration maintained by low unit doses of cocaine and cocaine (but not sucrose)-induced reinstatement of drug-seeking behavior. Microinjections of 2-PMPA (3-5 microg/side) or NAAG (3-5 microg/side) into the nucleus accumbens (NAc), but not into the dorsal striatum, also inhibited cocaine-induced reinstatement, an effect that was blocked by intra-NAc injection of LY341495, a selective mGluR2/3 antagonist. In vivo microdialysis demonstrated that 2-PMPA (10-100 mg/kg, i.p.) produced a dose-dependent reduction in both extracellular dopamine (DA) and glutamate, an effect that was also blocked by LY341495. Finally, pre-treatment with 2-PMPA partially attenuated cocaine-enhanced extracellular NAc DA, while completely blocking cocaine-enhanced extracellular NAc glutamate in rats during reinstatement testing. Intra-NAc perfusion of LY341495 blocked 2-PMPA-induced reductions in cocaine-enhanced extracellular NAc glutamate, but not DA. These findings suggest that 2-PMPA is effective in attenuating cocaine-induced reinstatement of drug-seeking behavior, likely by attenuating cocaine-induced increases in NAc DA and glutamate via pre-synaptic mGluR2/3s.

N-acetylaspartylglutamate (NAAG) inhibits intravenous cocaine self-administration and cocaine-enhanced brain-stimulation reward in rats

Pharmacological activation of group II metabotropic glutamate (mGlu2 and mGlu3) receptors inhibits reward-seeking behavior and/or rewarding efficacy induced by drugs (cocaine, nicotine) or natural rewards (food, sucrose). In the present study, we investigated whether elevation of brain N-acetylaspartylglutamate (NAAG), an endogenous group II mGlu receptor agonist, by the NAAG peptidase inhibitor 2-PMPA attenuates cocaine's rewarding effects, as assessed by intravenous cocaine self-administration and intracranial electrical brain-stimulation reward (BSR) in rats. Systemic administration of 2-PMPA (10, 30, 100 mg/kg, i.p.) or intranasal administration of NAAG (100, 300 microg/10 microl/nostril) significantly inhibited intravenous cocaine self-administration under progressive-ratio (PR), but not under fixed-ratio 2 (FR2), reinforcement conditions. In addition, 2-PMPA (1, 10, 30 mg/kg, i.p) or NAAG (50, 100 microg/10 microl/nostril) significantly inhibited cocaine-enhanced BSR, but not basal BSR. Pretreatment with LY341495 (1 mg/kg, i.p.), a selective mGlu2/3 receptor antagonist, prevented the inhibitory effects produced by 2-PMPA or NAAG in both the self-administration and BSR paradigms. In vivo microdialysis demonstrated that 2-PMPA (10, 30, 100 mg/kg) dose-dependently attenuated cocaine-enhanced extracellular dopamine (DA) in the nucleus accumbens (NAc). 2-PMPA alone inhibited basal NAc DA release, an effect that was prevented by LY341495. These findings suggest that systemic administration of 2-PMPA or intranasal administration of NAAG inhibits cocaine's rewarding efficacy and cocaine-enhanced NAc DA - likely by activation of presynaptic mGlu2/3 receptors in the NAc. These data suggest a potential utility for 2-PMPA or NAAG in the treatment of cocaine addiction.

Oral administration of the NAALADase inhibitor GPI-5693 attenuates cocaine-induced reinstatement of drug-seeking behavior in rats

We have recently reported that the endogenous mGlu2/3 agonist N-acetylaspartylglutamate (NAAG) and the N-acetylated-alpha-linked-acidic dipeptidase (NAALADase, a NAAG degradation enzyme) inhibitor 2-PMPA significantly inhibit cocaine self-administration and cocaine-induced reinstatement of drug-seeking behavior by attenuating cocaine-enhanced extracellular dopamine and glutamate in the nucleus accumbens. However, the poor oral bioavailability of NAAG and 2-PMPA limits their practical use in humans. In the present study, we investigated the effects of the orally active NAALADase inhibitor GPI-5693 and its enantiomers on cocaine-taking and cocaine-seeking behaviours. We found that oral administration of GPI-5693 (15, 30, 60 mg/kg, p.o.) did not significantly alter intravenous cocaine self-administration under fixed-ratio (FR2) reinforcement, but significantly inhibited cocaine-induced reinstatement of the extinguished drug-seeking behavior. This inhibition was blocked by pretreatment with LY341495, a selective mGlu2/3 receptor antagonist. Pretreatment with the same doses (15, 30, 60 mg/kg, p.o.) of GPI-16476 or GPI-16477, two enantiomers of GPI-5693, also inhibited cocaine-induced reinstatement similar to GPI-5693. In contrast, GPI-5693 altered neither oral sucrose self-administration nor sucrose-triggered reinstatement of sucrose-seeking behavior. These data suggest that orally effective NAAG peptidase inhibitors deserve further study as potential agents for the treatment of cocaine addiction.