Intracellular Metabotropic Glutamate Receptor 5 (mGluR5) Activates Signaling Cascades Distinct from Cell Surface Counterparts

Abstract

G-protein-coupled receptors are thought to transmit extracellular signals to the cytoplasm from their position on the cell surface. Some receptors, including the metabotropic glutamate receptor 5 (mGluR5), are also highly expressed on intracellular membranes where they serve unknown functions. Here, we show that activation of cell surface versus intracellular mGluR5 results in unique Ca(2+) signatures leading to unique cellular responses. Specifically, activation of either cell surface or intracellular mGluR5 leads to JNK, Ca(2+)/calmodulin-dependent protein kinase (CaMK), and cyclic adenosine 3',5'-monophosphate-responsive element-binding protein phosphorylation, whereas activation of only intracellular mGluR5 leads to ERK1/2 and Elk-1 phosphorylation. Using pharmacological and genetic approaches, the present findings support a role for CaMK kinase in mediating mGluR5-dependent cyclic adenosine 3',5'-monophosphate-responsive element-binding protein phosphorylation, whereas CaMKII is upstream of intracellular mGluR5-mediated Elk-1 phosphorylation. Consistent with models showing Elk-1 regulating cascades of gene expression, the known Elk-1 targets c-fos and egr1 were up-regulated following intracellular mGluR5 activation, whereas a representative non-Elk-1 target, c-jun, was not. These findings emphasize that glutamate not only serves as a neurotransmitter for cell surface receptors but, when transported into the cell, can also activate intracellular receptors such as mGluR5. Glutamate activation of intracellular mGluR5 serves an important role in the regulation of nuclear Ca(2+), transcriptional activation, and gene expression necessary for physiological processes such as synaptic plasticity.