The study of allosteric modulator sites at the cannabinoid CB1 receptor

UNCG Author/Contributor (non-UNCG co-authors, if there are any, appear on document)
Teresa S. Barber (Creator)
The University of North Carolina at Greensboro (UNCG )
Web Site:
Patricia Reggio

Abstract: Org 27569, Org 27759 and Org 29647 are the first discovered allosteric modulators of the cannabinoid CB1 receptor. These ligands are thought to bind to "accessory binding sites" at the receptor . Binding of the Org allosteric modulators has been shown to affect the affinities of various CB1 ligands, but to reduce the efficacy of these ligands. The goal of this research project was to understand at a molecular level, the origins of the effects produced by the Org allosteric modulators. The study was begun by performing AM1 conformational searches for each allosteric modulator using the Spartan molecular modeling suite . Those conformers within 2.00 kcal/mol of the global minimum energy conformer of each modulator were subjected to geometry optimization in Jaguar (Schrodinger, Inc). Org27569 was then targeted for further study. Org 27569 has been reported to increase the CB1 affinity of the non-classical cannabinoid, (1R3R4R)-3-[2-hydroxy-4-(1,1-dimethylheptylphenyl]-4-(3-hydroxy-propyl)cyclohexab-1-ol, CP-55,940, but to reduce its efficacy. Since the binding site of Org27569 is unknown, the MMC program was then employed to identify potential binding sites. The MMC program is a cavity biased method that uses Monte Carlo simulated annealing of chemical potential to identify small-molecule binding sites in protein structures via a molecular fragment approach [F. Guarnieri and M. Mezei,JACS 118, 8493, 1996]. The receptor was placed in a virtual cell. At high chemical potentials, the box is filled with completely with the fragment of interest. As the chemical potential decreases, fragments with less favorable interactions are stripped away. Indole and piperidine rings were used as fragments because they constitute the two major structural features of Org 27569. Three common binding sites for both the indole and piperidine fragments were identified. These areas were R3.50 (intracellular domain), W4.50 (possible homodimer interface) and in the transmembrane region between helices 1 and 2 (interacting with CP55,940). These were considered possible interaction sites for the following reasons: (1) If interactions occurred between the allosteric modulator and R3.50, this would block the interaction site of the G-protein and thus impair signalling. (2) W4.50 is commonly found in GPCR dimer interfaces. If the CB1 receptor functions as a dimer, Org27569 would impair activation by blocking dimer formation. (3) The TMH1-2 site site would allow the affinity of CP55940 to increase because it would block CP55940 exit from CB1. At the same time, the TMH1-2 site dock would constrain TMH6 from moving during activation by tethering the EC-3 loop. This should also result in impaired signal transduction. Future studies will involve mutation studies of each allosteric binding site identified in this project to determine the allosteric binding site for Org 27569.

Additional Information

Language: English
Date: 2009
Allosteric modulators, Cannabinoid CB1 receptor
Cannabinoids $x Receptors
Allosteric regulation

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