Decreased expression of the metabotropic glutamate receptor-4 gene is associated with neuronal apoptosis

Cultured cerebellar granule neurons die by apoptosis when switched from medium containing elevated potassium (K+) and serum to serum-free medium containing low K+. Although cell death begins at about 16 hr, commitment to death occurs within 6 hr after the lowering of K+. We have used this paradigm to examine the role of metabotropic glutamate receptor (mGluR) genes in the regulation of neuronal survival. We find that the expression of one of the mGluR genes, the type-4 gene, is associated with increased neuronal survival. Lowering of K+ leads to an 80% decrease in mGluR-4 mRNA expression within 6 hr. Downregulation of mGluR-4 messenger RNA (mRNA) does not occur if low K+-induced death is prevented by treatment with insulin-like growth factor I or adenosine 3',5'-cyclic monophosphate. If transcription is inhibited by actinomycin D, the difference in mGluR4 mRNA expression between cells switched to high-K+ medium and those switched to low-K+ medium is dramatically reduced, suggesting that decreased mGluR-4 gene transcription rather than increased mRNA breakdown is mainly responsible for the apoptosis-associated decrease in mGluR4 levels. Blockade of transcription also reduces mGluR4 mRNA expression in healthy neurons by more than 50% within 4 hr, suggesting that the mGluR4 mRNA has a relatively short half-life. In pharmacological experiments, we observe that the specific group III mGluR agonists such as L-amino-4-phosphobutyric acid and O-phospho-L-serine inhibit low K+-induced apoptosis. On the other hand, a selective mGluR4 antagonist, (RS)-alpha-cyclopropyl-4-phosphono-phenylglycine, induces apoptosis even in the presence of elevated K+. These results indicate that elevated mGluR4 expression or the activation of this receptor promotes survival and that an inhibition of such survival mechanisms contributes to apoptosis.