The Role of Intracellular Metabotropic Glutamate Receptor 5 at the Synapse

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DESCRIPTION (provided by applicant): The Role of Intracellular Metabotropic Glutamate Receptor 5 at the Synapse Project Summary By coupling to various intracellular cascades, metabotropic glutamate receptor 5 (mGluR5) plays an important role throughout the CNS in modulating neuronal activity and synaptic transmission. Dysfunction of mGluR5 is implicated in a variety of neurological problems including anxiety, seizures, addiction, learning and memory disorders, and Fragile X Syndrome (FXS). In FXS, a single-gene disorder characterized by mental retardation and a range of autistic features, an expanded trinucleotide repeat results in a deficiency of Fragile X Mental Retardation Protein (FMRP). FMRP normally acts as a translational repressor to oppose mGluR5 signaling; thus, the lack of FMRP in FXS leads to overactivity of mGluR5. To correct the excess mGluR5 activity, mGluR5 antagonists are currently undergoing clinical trials as a treatment for FXS. Although G-protein coupled receptors like mGluR5 are traditionally thought to initiate their signaling cascades from the cell surface, there is mounting evidence that intracellular receptors, located on the nuclear membrane and endoplasmic reticulum, are also physiologically significant. Notably, up to 90% of mGluR5 is intracellularly located, where it gives rise to unique calcium responses and downstream signaling cascades in dissociated striatal cultures. The native ligand of mGluR5, glutamate, can cross the cell membrane via various transporters and exchangers; therefore, glutamate released at a synapse can activate both cell surface mGluR5 as well as intracellular mGluR5. The downstream pathways initiated by mGluR5 activation lead to local protein synthesis at the synapse and AMPA receptor (an excitatory ionotropic glutamate receptor) endocytosis as well as transcription of synaptic plasticity genes; however, the contributions of cell surface mGluR5 versus intracellular mGluR5 are not yet well-established. The goal of this proposal is to test the hypothesis that activation of intracellular mGluR5 versus cell surface mGluR5 has differential effects on protein synthesis and AMPA receptor internalization in the hippocampus. The hippocampus is an appropriate area on which to focus due to its importance in learning and memory and its established responses to cell surface mGluR5 activation by the impermeable mGluR5 agonist, DHPG. Using sets of permeable and impermeable mGluR5 agonists and antagonists, activation of intracellular mGluR5 or cell surface mGluR5 can be achieved in isolation in order to delineate their downstream effects; for instance, intracellular receptors alone can be activated with application of a permeable agonist and an impermeable antagonist. Defining the role of intracellular mGluR5 activation is important both conceptually, in showing the physiological relevance of intracellular G-protein coupled receptors, and clinically, in targeting drugs to intracellular receptors, cell surface receptors, or both. For Fragile X Syndrome and possibly autism, understanding the role of intracellular mGluR5 will be crucial in developing suitable mGluR5 antagonists for therapeutic treatments. PUBLIC HEALTH RELEVANCE: This project will benefit public health in the search for appropriate treatments for Fragile X Syndrome by improving our understanding of the underlying mechanisms of this disorder. This research will help clarify the function of a receptor known to be involved in Fragile X Syndrome. The knowledge gained from these studies may be applicable to autism spectrum disorders as well, which would have a major impact on public health given the recent increase in incidence of autism.