The Neuroprotective Activity of Group-II Metabotropic Glutamate Receptors Requires New Protein Synthesis and Involves a Glial–Neuronal Signaling

Abstract

The group-II metabotropic glutamate (mGlu) receptor agonists (2S,1′R,2′R,3′R)-2-(2,3-dicarboxycyclopropyl)glycine (DCG-IV), S-4-carboxy-3-hydroxyphenylglycine (4C3HPG), and (2S,1′S,2′S)-2-(carboxycyclopropyl)glycine (L-CCG-I) protected mouse cortical neurons grown in mixed cultures against excitotoxic degeneration induced by a 10 min pulse with NMDA. Protection was observed not only when agonists were added in combination with NMDA but also when they were transiently applied to cultures 6–20 hr before the NMDA pulse. In both cases, neuroprotection was reduced by the group-II mGlu receptor antagonist (2S,1′S,2′S,3′R)-2-(2′-carboxy-3′-phenylcyclopropyl)glycine (PCCG-IV), as well as by the protein synthesis inhibitor cycloheximide (CHX). Both neurons and astrocytes in mixed cultures were immunostained with an antibody that recognized mGlu2 and mGlu3 receptors in recombinant cells. To determine whether astrocytes played any role in the neuroprotection mediated by group-II mGlu receptors, we exposed pure cultures of cortical astrocytes to DCG-IV, 4C3HPG, or L-CCG-I for 10 min. The astrocyte medium collected 2–20 hr after the exposure to any of these drugs was highly neuroprotective when transferred to mixed cultures treated with NMDA. This protective activity was reduced when CHX was applied to astrocyte cultures immediately after the transient exposure to group-II mGlu receptor agonists. We conclude that neuroprotection mediated by group-II mGlu receptors in cultured cortical cells requires new protein synthesis and involves an interaction between neurons and astrocytes.

The Use of Knock-Out Mice Unravels Distinct Roles for mGlu2 and mGlu3 Metabotropic Glutamate Receptors in Mechanisms of Neurodegeneration/Neuroprotection

“…This offers the advantage to examine the respective contribution of astrocytic and neuronal mGlu2 and mGlu3 receptors to processes of neurodegeneration/neuroprotection. Neuroprotection by mGlu2/3 receptor agonists in mixed cortical cultures involves a mechanism of glial-neuronal interaction mediated by the production of transforming growth factor-␤ (Bruno et al, 1997(Bruno et al, , 1998bD'Onofrio et al, 2001). This mechanism was dem- onstrated by transferring the medium collected from astrocyte cultures treated with mGlu2/3 receptor agonists to recipient mixed cultures challenged with NMDA (Bruno et al, 1997).…”

“…Neuroprotection by mGlu2/3 receptor agonists in mixed cortical cultures involves a mechanism of glial-neuronal interaction mediated by the production of transforming growth factor-␤ (Bruno et al, 1997(Bruno et al, , 1998bD'Onofrio et al, 2001). This mechanism was dem- onstrated by transferring the medium collected from astrocyte cultures treated with mGlu2/3 receptor agonists to recipient mixed cultures challenged with NMDA (Bruno et al, 1997). Using the same strategy, we have now shown that the mechanism of glial/neuronal interaction is mediated by mGlu3 receptors and that activation of astrocytic mGlu3 receptors is responsible for neuroprotection induced by LY379268.…”

“…We have shown previously that activation of group II mGlu receptors in astrocytes promotes neuroprotection via a paracrine mechanism mediated by TGF-␤ and perhaps other neurotrophic factors (Bruno et al, 1997(Bruno et al, , 1998bCiccarelli et al, 1999;D'Onofrio et al, 2001). We therefore examined whether the medium collected from pure cultures of astrocytes prepared from wt or mGlu3 Ϫ/Ϫ mice treated or not with LY379268 affected neuronal death in recipient mixed cultures challenged with NMDA prepared from a-wt/n-wt or a-mGlu3…”

“…One hypothetical advantage associated with the use of mGlu receptor ligands is the lack of the adverse effects typically induced by NMDA or AMPA receptor antagonists, such as sedation, ataxia, and severe learning impairment Bruno et al, 2001). Of the eight known mGlu receptor subtypes, mGlu2 and mGlu3 receptors are the best candidates as "neuroprotective receptors," because their activation inhibits glutamate release (Lovinger, 1991;Lovinger and McCool, 1995;Battaglia et al, 1997;Cozzi et al, 1997), inhibits voltage-gated calcium channels (for review, see Pin and Duvoisin, 1995), positively modulates potassium channels (Sharon et al, 1997), and stimulates the production of neurotrophic factors in astrocytes and microglia (Bruno et al, 1997(Bruno et al, , 1998bCiccarelli et al, 1999;D'Onofrio et al, 2001;Matarredona et al, 2001). Early in vitro studies have shown that first generation agonists of mGlu2 and mGlu3, such as (2S,1ЈS,2ЈS)-2-(carboxycyclopropyl)glycine and (2S,2ЈR,3ЈR)-2-(2Ј,3Ј-dicarboxycyclopropyl)glycine, are protective against excitotoxicity and other neuronal insults (Pizzi et al, 1993;Bruno et al, 1994Bruno et al, , 1995Ambrosini et al, 1995;Buisson and Choi, 1995;Thomsen et al, 1996).…”

Activation of Microglial Group III Metabotropic Glutamate Receptors Protects Neurons against Microglial Neurotoxicity

“…Whether this neuroprotection is attributable to the inhibition of microglial toxin production or the stimulated release of additional neuroprotective substances remains to be determined. In astrocytes, mGlu receptor stimulation may lead to the release of neurotrophic factors, such as TGF-␤ (Bruno et al, 1997(Bruno et al, , 1998.…”