Molecular Light Shed on “Dark” Cellular Receptors

Scientists have created a new research tool to help find homes for orphan cell-surface receptors, toward better understanding of cell signaling, developing new therapeutics, and determining causes of drug side-effects. The approach may be broadly useful for discovering interactions of orphan receptors with endogenous, naturally occurring ligands or with drugs.

To probe the activity of orphan G protein-coupled receptors (GPCRs), researchers at the University of North Carolina School of Medicine (Chapel Hill, NC, USA) and University of California, San Francisco (UCSF; San Francisco, CA, USA) developed a technique that combines computer modeling, yeast cell-based molecular screening, and mouse models.

Chemical signals remain unknown for many cell receptors in the human genome. These “orphan” receptors are highly expressed in particular tissues but their functions remain a mystery. They are considered “dark” elements of the genome and yet hold great potential for cell biology and medical therapeutics. “About 27% of FDA-approved drugs act through GPCRs. They are considered to be among the most useful targets for discovering new medications,” said Brian Shoichet, PhD, co-senior author and professor of pharmaceutical chemistry, UCSF. “We provide an integrated approach that we believe can be applied to many other receptors,” said Bryan L. Roth, MD, PhD, co-senior author and professor of pharmacology, UNC School of Medicine.

In the study, the tool was used to identify molecules that can modulate the orphan GPCR called GPR68 (or OGR1), a proton receptor highly expressed in the brain hippocampus. The Roth lab teamed up with the Shoichet lab, which developed the computational method. The goal was to predict those very few molecules that could modulate GPR68. Docking 3.1 million molecules predicted modulators, many of which were confirmed in functional assays. The researchers also found that the molecule “ogerin” activates GPR68. To understand how this affects brain function, mice were given ogerin and put through a battery of behavioral tests. Mice given ogerin were much less likely to learn to fear a specific stimulus, a fear-conditioning controlled by the hippocampus. Ogerin had no effect on control GPR68-knockout mice.

Xi-Ping Huang, PhD, co-first author and research assistant professor, UNC, said, “We used yeast-based screening techniques to find compounds that activate an orphan receptor. 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 same approach led to discovery of allosteric agonists and negative allosteric modulators for another orphan receptor, GPR65, suggesting that the tool has general applicability for identifying GPCR ligands. The tool opens a new door for both basic and applied research. The genome is still “an iceberg that is mostly submerged,” said Prof. 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.”

The study, by Huang X-P, Karpiak J, et al., was published online ahead of print November 9, 2015, in the journal Nature.

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