Lead authors Kirill Martemyanov, Ph.D., and Brock Grill, Ph.D., describe how they designed and implemented a new, unbiased approach for decoding the genetic network that controls actions of opioids in a nervous system.
‘GPR139 could someday play a significant role in decreasing withdrawal symptoms in patients, making opioids safer to prescribe.’
They used a small soil-dwelling animal, the nematode worm, to discover something surprising about one of the most-studied drug receptors.
"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 says.
Their system relies upon the nematode c. elegans, engineered to express the mammalian surface receptor for painkilling drugs, the μ (mu) opioid receptor (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 were 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," Grill says. "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 in 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, Martemyanov says. The discovery could point a way toward lessening the suffering associated with opioid withdrawal, Grill says.
"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," Grill says.
Opioids, a group of drugs that include fentanyl, Vicodin, OxyContin, and morphine, are highly effective at blunting severe pain. However prolonged use can create tolerance and dependence. Excessive use can cause an overdose.