Neuroprotection mediated by glutamate carboxypeptidase II (NAALADase) inhibition requires TGF-β

Inhibition of glutamate carboxypeptidase (GCP) II (EC 3.4.17.21), also termed N-acetylated alpha-linked acidic dipeptidase (NAALADase), has been shown to protect against ischemic injury presumably via decreasing glutamate and increasing N-acetyl-aspartyl-glutamate (NAAG). NAAG is a potent and selective mGlu₃ receptor agonist. Activation of glial mGlu₃ receptors has been shown to protect against NMDA toxicity by releasing transforming growth factors, TGF-βs. We hypothesized that GCP II inhibition could be neuroprotective also via TGF-βs, due to increased NAAG. To verify this, Enzyme-Linked Immunosorbent Assays (ELISAs) were performed on media from both control and ischemic cultures treated with the GCP II inhibitor, 2-(phosphonomethyl)-pentanedioic acid (2-PMPA). We found that 2-PMPA attenuated ischemia-induced declines in TGF-β. To further assess the role of TGF-βs in 2-PMPA-mediated neuroprotection, a neutralizing antibody to TGF-β (TGF-β Ab) was used. In both in vitro and in vivo models of cerebral ischemia, TGF-β Ab reversed the neuroprotection by 2-PMPA. Antibodies to other growth factors had no effect. Data suggests that neuroprotection by GCP II inhibition may be partially mediated by promoting TGF-β release.