Unveiling the Opioid Receptors

People have used extracts of poppies to ease pain since the time of the Byzantine Empire. The active component, morphine, was isolated in 1804. And the receptors that mediate the effects of morphine and other opioids were identified in the early 1970s. At last, researchers know what the receptors look like, as two independent groups now report high-resolution crystal structures of not one, but two opioid receptors, μ (mu) and κ (kappa).

Brian Kobilka at Stanford University School of Medicine in California, US, and colleagues describe a structure of the μ receptor, the target of morphine and many other analgesic drugs, while a team led by Raymond Stevens at The Scripps Research Institute in La Jolla, California, US, tackle the κ receptor. Each receptor has a large ligand-binding pocket, and the researchers identify features that may mediate tight, selective binding, which could help drug developers find more potent, specific, and non-addictive painkillers. Both papers appeared online March 21 in Nature.

“Having these crystal structures is gigantic…. It just blows open the field,” said Michael Bruchas of Washington University, St. Louis, US, who studies opioid receptor signaling but was not involved in the new studies. “In the opioid field, it’s probably one of the biggest discoveries in a couple of decades,” he told PRF. “Is this going to start a new era in opioid pharmacology? I would like to believe yes.”

Opioids have unmatched effectiveness, but their side effects are legion. Morphine and other μ-targeted drugs can cause nausea, constipation, respiratory depression, sleepiness, dependence, and abuse. Compounds that bind the κ receptor are candidates for relieving pain without the risk of addiction, but they introduce other problems, especially depression and hallucinations. The problem is that opioid receptors are present throughout the central and peripheral nervous systems (and GI tract) and have many biological roles, and existing drugs are poorly selective. To avoid side effects, researchers and drug makers are aiming to make the compounds selective for particular receptor subtypes and downstream signaling pathways.

In their attempts to better target opioid receptors, however, researchers have been flying blind: While structures exist for a few receptors in the vast G protein-coupled receptor (GPCR) family, none are opioid receptors.

The new structures display the canonical GPCR bundle of seven membrane-crossing helices. One striking feature of both the μ and κ receptors is the large ligand-binding pocket on the extracellular side. The Kobilka team, co-led by Sébastien Granier, previously a member of Kobilka’s lab and now at INSERM in Montpellier, France, crystallized the mouse μ-opioid receptor with a morphine-like irreversible antagonist, β-funal-trexamine (β-FNA). The Stevens group crystallized the human κ receptor with the small-molecule antagonist JDTic, which is in early-stage clinical testing as a potential treatment for depression, anxiety, and drug addiction. Both compounds are receptor subtype selective and tight binders, and the structures clearly depict their points of contact. In addition, the investigators used sequence comparisons, mutational analysis, and computational binding simulations to identify structural features that may determine ligand selectivity for μ, κ, or another opioid receptor, δ (delta).

The roomy binding pockets offer myriad spots where drugs might attach and alter receptor activity in distinct ways. Having the structures will streamline efforts to identify new ligands, said F. Ivy Carroll and S. Wayne Mascarella, whose team at Research Triangle Institute, North Carolina, US, developed JDTic, and who are both authors on the κ structure paper. The new findings make possible virtual screening to sift through hundreds of thousands of compounds, they told PRF.

One caveat is that the current structures may not be exactly those that drug developers need to target. In both structures, antagonists freeze the receptors in inactive states, whereas pain-relieving drugs activate the receptors. So Kobilka says the current work is just the beginning. “We’re going to have to get other structures—structures in active states,” he told PRF.

In fact, opioid receptors do not just toggle between “on” and “off” conformations, but exist in many configurations, Kobilka said. Researchers are realizing that ligands can bind in “biased” or “functionally selective” ways, meaning that drugs could catch the receptors in just the conformation that elicits the desired downstream events (see PRF related news story, and for a review, see Raehal et al., 2011). Kobilka hopes to determine structures with ligands that favor various signaling pathways to help medicinal chemists develop more functionally selective drugs. In particular, he said, visualizing binding of the receptors’ natural activators—peptides such as endorphins—could suggest how to design synthetic compounds that activate the μ receptor without being addicting.

The structures may also inspire drug developers to look outside the binding pockets. In both the μ and κ crystals, the proteins associate intimately in dimers, lending support to existing notions that opioid receptors function in complexes. Thus far, Bruchas said, ligand development has operated largely on the assumption that the receptor is a monomer, so tweaking the formation of multiprotein complexes might be a fruitful route to targeting the receptor. “I think seeing it as a dimer is going to turn some heads,” he said.

Top image: The binding pocket of the μ-opioid receptor, with a morphine-like ligand. Credit: Kobilka lab.