Neuroprotection afforded by NAAG and NAALADase inhibition requires glial cells and metabotropic glutamate receptor activation

Abstract

N-acetylated-α-linked-acidic-dipeptidase (NAALADase or glutamate carboxypeptidase II) cleaves the neuropeptide N-acetyl-aspartyl-glutamate (NAAG) to glutamate and N-acetyl-aspartate (NAA). Previously, NAAG and 2-(phosphonomethyl)-pentanedioic acid (2-PMPA), a potent and selective NAALADase inhibitor, were found to be neuroprotective in neuronal/glial co-cultures and in animals following transient middle cerebral artery occlusion. In this report, we examined the involvement of glial cells and metabotropic glutamate (mGlu) receptors in neuroprotection mediated by NAAG and 2-PMPA in an in vitro model of metabolic inhibition. Neuroprotection of neuronal/glial co-cultures by both NAAG and 2-PMPA, against metabolic inhibition, was significantly higher than neuroprotection in the absence of glia. Similarly, (2 S,2′ R,3′ R)-2-(2′,3′-dicarboxycyclopropyl)glycine (DCG IV), a selective group II mGlu receptor agonist, was less neuroprotective in the absence of glia. Selective group II mGlu receptor antagonists and ( S)-α-methyl-4-carboxyphenylglycine (MCPG), a non-selective mGlu receptor antagonist, reduced the protection afforded by both NAAG and 2-PMPA when using neuronal/glial co-cultures. In contrast, groups I and III mGlu receptor antagonists did not affect NAAG or 2-PMPA neuroprotection. These results underscore the critical involvement of glia and group II mGlu receptors in NAAG and 2-PMPA-mediated neuroprotection.