| Previously, we have shown that mGlu1 receptors play a dual role in neuronal apoptosis: mGlu1 receptor overexpression led to massive cell death while several mGlu1 receptor agonists protected neurons from this receptor-induced apoptosis. In this study, we investigated the possible role of mGlu1 receptor C-terminal domain in receptor induced toxicity. Using stably mGlu1 transfected CHO cells, we observed the cleavage of mGlu1 C-terminal domain when cells were deprived from serum. This cleavage was partially prevented by caspase inhibitors, and expression of an mGlu1 C-terminal domain construct in primary cultures of cerebellar neurons induced toxicity. We also used primary cultures of cerebellar granule neurons as a model of neuronal apoptosis and survival. These neurons survive and differentiate well in a medium supplemented with high potassium concentrations (25 mM, K25 conditions) while physiological K+ concentrations (5 mM, K5 conditions) lead to an increasing in mGlu1 receptor expression and robust cell death. Under these conditions K5 toxicity was blocked by the mGlu1 receptor selective antagonists CPCCOEt and JNJ. This suggests a role for mGlu1 receptors in cell toxicity during neuronal development and low K5 conditions may be regarded as an in vitro model of developmental apoptosis. Our previous observations have also indicated that under these conditions enhanced mGlu1 expression is influenced by the E2F1 transcription factor, suggesting the involvement of proteins associated with the cell cycle. In fact, transferring granule cells from K25 to K5 conditions caused a differential, time-dependent, regulation of several such proteins. Under these conditions, we also observed enhanced DNA synthesis, as measured by BrdU incorporation. Moreover, a selective cdk2 inhibitor reversed K5 toxicity, while selective cdk1 and cdk4/6 inhibitors failed to provide this protective effect. These results indicate that mGlu1 receptors are involved in a complex manner in the apoptosis of cerebellar neurons which may result from an aberrant reentry into the cell cycle which leads to apoptotic death of post-mitotic cells. The interactions between mGlu1 receptors and cell cycle proteins may involve both negative, agonist-independent, and positive, glutamate-mediated, signaling of mGlu1 receptors. |
