N-Acetylated α-Linked Acidic Dipeptidase Converts N-Acetylaspartylglutamate from a Neuroprotectant to a Neurotoxin
Abstract
We previously reported that inhibition of the brain enzymeN-acetylated α-linked acidic dipeptidase (NAALADase; glutamate carboxypeptidase II) robustly protects cortical neurons from ischemic injury. Since NAALADase hydrolyzesN-acetylaspartylglutamate (NAAG) to glutamate we hypothesized that inhibiting NAALADase would both decrease glutamate and increase NAAG. Increasing NAAG is potentially important because NAAG is a metabotropic glutamate receptor agonist and anN-methyl-d-aspartate (NMDA) partial antagonist, both of which have previously been shown to be neuroprotective. To understand the likely effects of endogenous NAAG in the central nervous system, we have now investigated the activity of NAAG in primary cortical cultures while manipulating NAALADase activity. Under hydrolyzing conditions, when NAALADase was active, NAAG had toxic effects that were blocked by NMDA and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate receptor antagonists and by NAALADase inhibition. NAAG's toxic effects were presumably due to the liberation of glutamate. Under nonhydrolyzing conditions, when NAALADase was inhibited, NAAG demonstrated neuroprotective effects against both NMDA toxicity and metabolic inhibition. In the case of NMDA-induced toxicity, NAAG provided neuroprotection through its partial antagonist activity at the NMDA receptor. In the case of metabolic inhibition, NAAG had an additional neuroprotective effect mediated through its agonist properties at the type II metabotropic glutamate receptor. These results indicate that NAAG might play an important role in the central nervous system, under certain pathological conditions, as a neurotoxin or as a neuroprotectant, depending on the activity of NAALADase.
Footnotes
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Send reprint requests to: Dr. Barbara Stauch Slusher, Ph.D., Guilford Pharmaceuticals Inc., 6611 Tributary St., Baltimore, MD 21224. E-mail: slusher_b{at}guilfordpharm.com
- Abbreviations:
- NAAG
- N-acetylaspartylglutamate
- NMDA
- N-methyl-d-aspartate
- NAALADase
- N-acetylated α-linked acidic dipeptidase
- mGluR
- metabotropic glutamate receptor
- 2-PMPA
- 2-(phosphonomethyl)-pentanedioic acid
- MEM
- minimum essential medium
- HBSS
- HEPES buffered saline solution
- LDH
- lactate dehydrogenase
- 2-DG
- 2-deoxyglucose
- CNQX
- 6-cyano-2,3-dihydroxy-7-nitroquinoxaline
- MCPG
- (+)-α-methyl-4-carboxyphenylglycine
- EGLU
- (2S)-α-ethylglutamic acid
- AIDA
- (R,S)-1-aminoindan-1,5-dicarboxylic acid
- MSOP
- (R,S)-α-methylserine-O-phosphate
- AMPA
- α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid
- NAA
- N-acetylaspartate
- MK-801
- (+)-MK801 hydrogen maleate
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- Received February 1, 2000.
- Accepted June 30, 2000.
- The American Society for Pharmacology and Experimental Therapeutics
