Unraveling the Mysteries of “Dark” Cellular Receptors

A research team from the University of North Carolina School of Medicine (UNC) and University of California, San Francisco (UCSF) says it has created a general tool to probe the activity of orphan receptors, which are highly expressed in particular tissues but whose functions remain unknown. The team intends to illuminate the roles of orphan receptors in behavior and make them accessible for drug discovery.

The creation of the research tool, which involves computer modeling, yeast- and mammalian cell-based molecular screening techniques, and mouse models, is published in an article (“Allosteric ligands for the pharmacologically dark receptors GPR68 and GPR65”) in Nature.

According to Brian Shoichet, Ph.D., professor of pharmaceutical chemistry at UCSF, this work will help researchers learn how orphan receptors interact with molecules inside the body or with drugs. Specifically, the UNC and UCSF scientists used their new tool to find a novel probe molecule that can modulate the orphan G protein-coupled receptor 68 (GPR68, also known as OGR1), an orphan receptor that is highly expressed in the brain.

“GPCRs are the single most important family of therapeutic drug targets,” said Dr. Shoichet. “About 27 percent of FDA-approved drugs act through GPCRs. They are considered to be among the most useful targets for discovering new medications.”

The new probe molecule, dubbed “ogerin,” turns on GPR68, activating its signaling role. To understand how this activation of GPR68 affects brain function, the investigators gave it to mice and put them through a battery of behavioral tests. Mice that had been given ogerin were much less likely to learn to fear a specific stimulus. This fear-conditioning is controlled by the hippocampus, where GPR68 is highly expressed. But ogerin had no effect on mice that lacked this receptor.

To demonstrate general research applicability, the UNC and UCSF researchers also used their technique to find molecules that can modulate GPR65, another orphan receptor.

“We provide an integrated approach that we believe can be applied to many other receptors,” explained Bryan L. Roth, M.D., Ph.D., co-senior author of the Nature paper and the Michael Hooker Distinguished Professor of Protein Therapeutics and Translational Proteomics in the department of pharmacology at UNC. “The goal is to quickly find drug-like compounds for these receptors. This should facilitate discovery of novel and safer therapeutics for a host of diseases.”

“We used yeast-based screening techniques to find compounds that activate an orphan receptor,” noted Xi-Ping Huang, Ph.D., co-first author and research assistant professor at UNC. “Then [co-first author] Joel Karpiak, a graduate student in Shoichet's lab at UCSF, created a computer model and searched libraries of millions of compounds to find out what kind of molecular structure ensures proper binding and interaction with a specific receptor. Then, back in the lab, we tested new molecules and found a novel ligand.”

“The fact that GPR68 is highly expressed in several tissues, especially the brain, and that it is a member of the large GPCR family, suggests that this discovery can be further leveraged for drug discovery,” added Dr. Shoichet.

Currently, few drug developers would seek drugs for a target with an unknown role in human biology. With new evidence of the role of GPR68, and a molecule that can modulate that role, the door has been opened for further research studies, both basic and applied.

“The druggable genome is an iceberg that is mostly submerged,” pointed out Dr. Shoichet. “This paper illuminates a small piece of it, providing new reagents to modulate a previously dark, unreachable drug target. Just as important, the strategy should be useful to many other dark targets in the genome.”