Scientists at Scripps Research in Florida report that they have discovered a biological system that manages cells’ response to opioid drug exposure. The team said its discovery offers new ideas for improving the safety of the one of the most effective, and most abused, group of pain medications.
In a paper (“Genetic behavioral screen identifies an orphan anti-opioid system”) published in Science, lead authors Kirill Martemyanov, PhD, and Brock Grill, PhD, described how they designed and implemented a new approach using a nematode worm for decoding the genetic network that controls actions of opioids in a nervous system.
“Opioids target μ-opioid receptor (MOR) to produce unrivaled pain management but their addictive properties can lead to severe abuse. We developed a whole animal behavioral platform for unbiased discovery of genes influencing opioid responsiveness. Using forward genetics in C. elegans, we identified a conserved orphan receptor, GPR139, with anti-opioid activity,” the investigators wrote. “GPR139 is coexpressed with MOR in opioid-sensitive brain circuits, binds to MOR and inhibits signaling to G proteins. Deletion of GPR139 in mice enhanced opioid-induced inhibition of neuronal firing to modulate morphine-induced analgesia, reward, and withdrawal. Thus, GPR139 could be a useful target for increasing opioid safety. These results also demonstrate the potential of C. elegans as a scalable platform for genetic discovery of GPCR signaling principles.”
“A study like this makes it clear that even though we may think we know everything there is to know about the opioid response, we’re actually just scratching the surface,” Martemyanov said.
Their system relies upon C. elegans, engineered to express the mammalian surface receptor for painkilling drugs, MOR. The receptor is not normally found in the worms’ DNA, and adding it made the transgenic animals respond to opioids like morphine and fentanyl. The researchers then exposed the worms to mutagens and selected the ones with abnormal responses to opioids. Whole-genome sequencing and CRISPR engineering was then used to pinpoint the genes responsible for those aberrant responses.
“Forward genetics—unbiased genetic discovery—has never been applied to probing an opioid receptor like this,” noted Grill. “The opioid epidemic is a huge problem and we don’t have good solutions. This type of approach can bring a whole new array of targets and a new way of thinking about and going after an old problem.”
The work ultimately led the researchers to the worms’ FRPR-13 receptor, conserved in all animals, and known as GPR139 in mammals. It is considered an “orphan” G-protein coupled receptor (GPCR) with poorly understood biology and unknown role in physiology. Further studies in mice showed that GPR139 was expressed on the same neurons as MOR and counteracted the effects of opioids on neuronal firing.
When researchers administered drugs that activate GPR139, mice dependent on opioid intake stopped taking the drug. Conversely, genetic elimination of GPR139 augmented the pain-killing effects of opioids. The genetically modified mice lacking GPR139 also showed something remarkable—they showed very minimal withdrawal symptoms following chronic exposure to opioids. Withdrawal syndrome, a set of extremely unpleasant symptoms, usually sets in upon the discontinuation of opioids following their prolonged use. This compels people to resume drug-taking, fueling the dependence, according to Martemyanov. The discovery could point a way toward lessening the suffering associated with opioid withdrawal, added Grill.
“A lot of addicts know that if they stop using, they are going to deal with anxiety, nausea, tremor, and they are going to be in a lot of pain. That probably has a very negative impact on people wanting to go into rehab,” pointed out Grill.