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Method for identifying ligands activating either excitatory or inhibitory G-protein-coupled receptors by functional
1Department of Physiology and Neuroscience, Wallenberg Neuroscience Center, University of Lund, Sweden. christer.owman@mphy.lu.se
Abstract:
Xenopus oocytes devoid of their follicular enclosure provide a frequently used expression system for investigating receptors that transduce through activation of adenylyl cyclase following injection of the appropriate mRNA. However, due to a low basal activity of the cyclase they cannot be utilized to investigate receptor-mediated reductions in endogenous cAMP levels. In order to overcome this limitation, a model was designed in which test clones for such inhibitory receptors were co-expressed with a beta 2-adrenoceptor, which elevated cAMP upon exposure to isoproterenol. Following injection of mRNA to express the alpha 2 test receptor in the oocytes, marked reduction in cAMP could be measured after exposure to clonidine. Attenuation of cAMP levels was also seen following co-expression of the dopamine D2 receptor along with dopamine administration. Thus, after inducing a receptor-mediated tone in adenylyl cyclase activity, Xenopus oocytes can be conveniently used to study also ligands that bind to inhibitory G-protein coupled receptors.
Insights
Xenopus oocytes can now study inhibitory receptors by co-expressing a beta 2-adrenoceptor to elevate cAMP. This method enables measurement of receptor-mediated reductions in cyclic adenosine monophosphate (cAMP) levels, expanding their use in drug discovery.
Area of Science:
- Pharmacology
- Molecular Biology
- Cellular Biology
Background:
- Xenopus oocytes are a common expression system for receptors linked to adenylyl cyclase activation.
- Low basal adenylyl cyclase activity limits their use for studying inhibitory receptors that reduce cyclic adenosine monophosphate (cAMP).
Purpose of the Study:
- To develop a method for studying inhibitory G-protein coupled receptors (GPCRs) in Xenopus oocytes.
- To overcome the limitation of low basal adenylyl cyclase activity in oocytes.
Main Methods:
- Co-expression of inhibitory test receptors (e.g., alpha 2, dopamine D2) with a beta 2-adrenoceptor in Xenopus oocytes.
- Induction of a cAMP 'tone' using the beta 2-adrenoceptor and isoproterenol.
- Measurement of cAMP level reductions upon ligand administration to the test receptors.
Main Results:
- Successful measurement of clonidine-induced cAMP reduction with co-expressed alpha 2 receptors.
- Observed dopamine-induced cAMP attenuation with co-expressed dopamine D2 receptors.
- Demonstrated the feasibility of studying inhibitory GPCRs in Xenopus oocytes.
Conclusions:
- Xenopus oocytes can be adapted to study ligands binding to inhibitory GPCRs.
- This enhanced model system facilitates research on receptors that decrease cAMP levels.
- The method expands the utility of Xenopus oocytes for pharmacological research.