Karen L. O'Malley, PhD
Principal Investigator
Carolyn Ann Hogan, Graduate Student (2010)
Vikas Kumar, PhD, Postdoctoral Fellow (2008-9)
Washington University School of Medicine
St. Louis, MO
FRAXA Awards:
$32,500 in 2010
$60,000 in 2009
$60,000 in 2008
Many of the symptoms of Fragile X Syndrome (FXS) are thought to arise due to overactive metabotropic glutamate receptor 5 (mGluR5) signaling, which is normally opposed by the protein missing in FXS, Fragile X Mental Retardation Protein (FMRP). mGluR5 is an important neuronal receptor that controls proteins involved in modifying synapses, which are the bridges by which neurons communicate with one another. Through its involvement in remodeling synapses, mGluR5 has been implicated as a key player in many neurological processes, such as learning and memory. Recent advances investigating mGluR5 activity in FXS have led to the exciting results that drugs that inhibit mGluR5 activity (called mGluR5 antagonists) may be useful in treating the behavioral symptoms of FXS. Currently, clinical trials are underway to further study the safety and efficacy of mGluR5 antagonists, such as fenobam, in the treatment of FXS.
Although receptors like mGluR5 are traditionally thought to initiate their signaling cascades from the cell surface, there is mounting evidence that intracellular receptors, located on membranes inside cells, are also physiologically active. However, studies of mGluR5 up to this point have focused solely on cell surface signaling, despite the fact that that up to 90% of mGluR5 is intracellularly located. To remedy this, our research concentrates on discovering the properties and functions of intracellular mGluR5 as compared to cell surface mGluR5. In additional, it is also essential to test the permeability of mGluR5 antagonists in order to determine whether they are acting at the cell surface mGluR5 alone or if they reach the inside of cells as well. By analyzing the roles of intracellular and cell surface mGluR5 in FXS and determining the permeability properties of mGluR5 antagonists, we hope to achieve increased specificity in targeting drugs to the appropriate cellular locations and thus improved treatment of FXS.
By Karen O'Malley and Vikas Kumar, 6/2008
Typically G-protein coupled receptors such as the metabotropic glutamate receptor, mGlu5, are activated by contact with their neurotransmitters at the cell surface. Although receptors are also found within the cell they are presumed to be receptors that have just come off the cell surface or receptors poised to be inserted into the membrane if needed. Interestingly, however, a number of receptors have been found that can move from the cell surface all the way into the nucleus and there are other receptors that seem to be primarily expressed within the cell. What are these intracellular receptors doing? Is it the same function as the cell surface receptors? How are their neurotransmitters reaching them? What are the signals that send these receptors to different sites within the cell? Are there special proteins that hold intracellular receptors in place? And, importantly for various human diseases, can we design drugs that will target separate populations of receptors i.e. just the cell surface versus cell surface or just intracellular?
These are a few of the questions we have been focusing on in the last few years using mGlu5 as a model. To date, we know that in mGlu5-expressing striatal neurons, activation of intracellular receptors can lead to a much more robust and longer lasting signaling response than cell surface receptors. We know that the neurotransmitter can reach intracellular receptors using well-known transporter proteins. We also know that neurotransmitter activation of intracellular receptors can activate overlapping as well as distinct signaling systems within the neuron leading to the activation of distinct target genes. One differentially activated target gene in striatal neurons is the fragile X protein, FMRP.
As first described by Drs. Bear, Huber and colleagues, the mGlu5 receptor theory of fragile X syndrome theorizes that activation of mGlu5 is normally balanced by FMRP suppressing its responses. In fragile X syndrome, this balance is lost, leaving mGlu5 function unchecked. In most of these studies mGlu5 receptors are activated by the agonist, DHPG. Interestingly, DHPG is one of the drugs that we have shown can not cross the cell membrane. Thus DHPG only activates cell surface receptors. In vivo, however, it is glutamate release that activates receptors and glutamate can cross the cell membrane to activate intracellular receptors too. Therefore, our proposal is to test the idea that activation of cell surface versus intracellular receptors leads to differential changes in measures relevant to fragile x syndrome such as protein synthesis, AMPA receptor internalization and FMRP synthesis and localization. Results from these studies would further support our findings that depending upon their ability to cross cellular membranes, agonists and antagonists have differential effects on cell surface versus intracellular receptors. Most importantly, ligands might be tailored for even more selective applications based on the cellular distribution of the receptor.
