Research Overview: G Protein Signaling

Goals: Elucidate novel G protein signaling mechanisms that are likely to have key roles in cardiovascular or nervous system disorders, or in cancer. Toward this goal we are working to:

Significance: G protein signaling is central to the etiology and treatment of: 1) cardiovascular diseases including heart failure and hypertension; 2) nervous system disorders such as depression, schizophrenia and drug addiction; and 3) cancer. As new G protein signaling mechanisms are elucidated by basic research, novel disease mechanisms will be revealed and new therapeutics developed. Further background information about G protein signaling can be found at the STKE web site and GPCR database.

Current projects (also see page devoted to lab people):

Techniques:

Project Summaries

G protein signaling in hypertension: We have discovered a genetic defect in a GPCR regulatory mechanism that causes hypertension in mice, and that has been linked to hypertension in certain human populations. We have found that a G protein regulator called RGS2 is essential for blood pressure regulation. We showed that RGS2 knockout mice are strikingly hypertensive. Even RGS2-/+ mice exhibit elevated blood pressure. This is very exciting because it shows that RGS2 is a hypertension QTL (quantitative trait locus). We therefore are in a unique position to establish the mechanism by which a hypertension QTL regulates blood pressure, long a goal of the field. This has led us to find that RGS2 is an effector of the nitric oxide (NO)-cGMP pathway that promotes relaxation of the resistance vasculature. Thus, without RGS2 the resistance vasculature does not relax normally, providing one mechanism contributing to hypertension. This also is an important discovery because the NO field has long been searching for downstream effectors that mediate vascular relaxation and regulate blood pressure.

Currently, we are working to understand the signaling mechanisms by which RGS2 promotes relaxation via the NO-cGMP pathway. These studies involve analysis of RGS2 expression, localization and activity.

Because the NO pathway has many other cardiovascular functions aside from promoting vascular relaxation, we are also investigating whether RGS2 functions as an NO effector in other organ systems. These experiments involve further analysis of cardiovascular function in wild type and RGS2 knockout mice, and will eventually employ studies using the relevant populations of primary cells. (Click here to return to the top of this page)

G protein signaling in the nervous system: The RGS7 family (RGS6, 7, 9 and 11) of G protein regulators is critically important for signaling in the brain and the visual system. For example, RGS9 knockouts exhibit augmented action of cocaine and opioids, as well as poor light adaptation and impaired contrast detection by the visual system. However, the mechanistic functions of RGS9 and other RGS7 family members are poorly understood, and the mechanisms that regulate the RGS7 family are not clear. Nevertheless, the RGS7 family probably has novel signaling and regulatory functions because they associate tightly with a novel G protein beta subunit (Gbeta5).

We believe that important clues regarding novel functions of the RGS7 family and Gbeta5 will be revealed by studies of R7BP, a novel RGS7-family binding protein we recently cloned. Our work suggests that R7BP functions as a shuttling protein that traffics RGS7/Gbeta5 complexes between the plasma membrane and nucleus. Shuttling is regulated by reversible palmitoylation/depalmitoylation of R7BP, which potentially provides a novel mechanism for transmitting neuronal GPCR signals directly from the plasma membrane to the nucleus.

Currently, we are working to understand how R7BP regulates the RGS7/Gbeta5 complex, how R7BP palmitoylation is regulated, what R7BP does in the nucleus, and how R7BP regulates neuronal GPCR signaling. We believe that these studies will reveal exciting new mechanisms that regulate neurotransmitter action, which are likely to be relevant to several neurological disorders and possibly drug addiction. (Click here to return to the top of this page)

Novel small molecule inhibitors of G protein signaling: Our studies of R7BP palmitoylation provide us with a novel opportunity to identify small molecule inhibitors of palmitoylation. Such inhibitors would provide the first pharmacological probes of palmitoylation function in G protein signaling in neurons and eventually in living animals. Given the importance of palmitoylation in a host of neuronal signaling pathways, palmitoylation inhibitors will be investigated as potentially novel agents for treating a variety of neurolgical diseases in which palmitoylated signaling proteins are likely to play important roles.

Currently, we are collaborating with Dr. Maurine Linder's lab in our department to develop a novel high-throughput screening system for palmitoyation inhibitors that initially utilizes our institutional screening facility. As this system is developed, we will then collaborate with the NIH screening center at Vanderbilt to conduct large scale screens for palmitoylation inhibitors. As lead compounds are identified, we will probe their activity in neuronal cell signaling in vitro to assess their specificity and efficacy. The next step will be to collaborate with chemists to conduct structure-activity studies that identify the pharmacophore responsible for palmitoylation inhibition, which will allow us to identify second generation inhibitors with greater specificity and potency. Over the long term, we will use these palmitoylation inhibitors to characterize the signaling processes these molecules target in neurons and in mice. (Click here to return to the top of this page)

Augmenting anti-Ras drug function in breast cancer: The G protein Ras has a critical role in many cancers, including breast cancer. Ras must be modified with a farnesyl lipid to associate with the plasma membrane and exert its tumor promoting effects. Therefore, drugs that inhibit farnesyltransferase (FTIs) have been developed by several companies and have exhibited limited, but promising, activity in breast cancer therapy. The major problem that limits the use of FTIs is that most breast cancer tumors are insensitive to these drugs for unknown reasons. Determining whether insensitivity occurs because FTIs fail to inhibit farnesylation or because other signaling pathways compensate for inhibition of Ras function is required to identify tumors likely to be FTI-sensitive or to develop novel means of augmenting FTI action in breast cancer.

Currently, we are working with the lab of Dr. David Piwnica-Worms at our institution to devise a novel bioluminescence imaging system that will enable investigators for the first time to monitor FTI action in tumors in living mice. With this system and small molecule and siRNA-based screens, we will endeavor over the long term to identify the mechanisms responsible for FTI insensitivity, with the ultimate goal of finding new ways of augmenting the action of FTIs in breast cancer therapy. (Click here to return to the top of this page)