The toll from opioid overdose in the United States now exceeds 45,000 deaths per year. Shockingly, more Americans die from opioid overdose than from motor vehicle collisions (1), and opioid overdose has become the number one cause of accidental death. Worldwide, two-thirds of drug-related deaths were a result of opioids, as reported by the United Nations 2019 drug report. As well as searching for opioid replacements, scientists are developing therapeutics to block the detrimental side effects of opioids, particularly addiction and fatal opioid-induced respiratory depression (OIRD) (2). Addiction and OIRD are a direct result of opioid activation of receptors that regulate neural circuits that control reward and breathing—circuits distinct from those that regulate pain (3). On page 1267 of this issue, Wang et al. (4) identify the orphan G protein–coupled receptor (GPCR) GPR139 as a regulator of opioid receptors and provide evidence that this receptor could be a useful therapeutic target to reduce opioid side effects.

Wang et al. turned to a model organism with a simpler nervous system than that in mammals, the nematode worm Caenorhabditis elegans (5). They developed an ingenious forward genetic screen to identify mutations that affect opioid receptor function. Fentanyl, morphine, and other abused opioids primarily act on the µ-type opioid receptor (MOR). MOR is a GPCR present on the cell surface. Deletion of Oprm1, the gene that encodes MOR, in mice demonstrated that MOR is responsible for the analgesic effects of opioids as well as the harmful addiction and OIRD (67). In rodents and humans, opioids have motor effects, altering locomotion and muscle tension (89). C. elegans does not express MOR and is unresponsive to opioids. However, the authors found that following introduction of a transgene to express mammalian MOR (tgMOR), fentanyl and morphine decreased locomotion in tgMOR C. elegans mutants. Wang et al. then induced random mutations in the tgMOR C. elegans population to identify worms resistant to fentanyl. They found ∼900 mutations and chose to examine one of the affected genes, frpr-13, which encodes a GPCR.

The mammalian ortholog of FRPR-13 is GPR139. Wang et al. validated the functional interaction of MOR and GPR139 by showing that MOR-induced membrane hyperpolarization, a known effect of opioids, was inhibited by GPR139 expression in cultured human kidney cells that do not usually express the GPCR. They next explored three possible molecular mechanisms by which GPR139 negatively regulates MOR function (see the figure). First, they explored whether GPR139 and MOR dimerization could explain this finding. They showed that GPR139 and MOR can be coimmunoprecipitated. However, both receptors were artificially overexpressed in a non-native cellular context; whether these interactions occur in vivo between endogenous receptors expressed at physiological concentrations and in the neurons that mediate opioid side effects remains to be determined. Second, when GPR139 is expressed at high amounts, they found that MOR is present at the cell surface in lower densities, suggesting that GPR139 may modulate Oprm1 expression and/or MOR trafficking inside the cell, either its transport to the cell surface or its internalization. At stoichiometric amounts, GPR139 has no effect on surface localization of MOR, suggesting that this mechanism may only take place when GPR139 expression is upregulated, potentially in the setting of chronic drug exposure and disease. Third, the authors provided evidence that GPR139 negatively regulates MOR by facilitating β-arrestin 2 recruitment. β-Arrestins are cytosolic proteins that interact with GPCRs and promote receptor desensitization, internalization, trafficking, and signaling (10). β-Arrestin 2 is thought to contribute differentially to the multiple side effects of opioids (e.g., promoting OIRD, reducing reward without affecting withdrawal) (1112) and may do so in an opioid ligand–dependent manner (13). Consequently, the exact opioid drugs and detrimental effects for which GPR139 modulation might prove useful will need to be established.

To investigate GPR139 function in neural circuits, the authors examined the medial habenula (MHb) and locus coeruleus (LC) of mice, two regions of the brain with neurons that express MOR, for electrophysiological evidence of MOR and GPR139 functional interactions. They found Oprm1 and Gpr139 mRNA colocalized in neurons in the MHb and LC. Using cultured brain slices, they found that the loss of Gpr139 reduced the basal firing rate of MHb neurons and increased the opioid sensitivity of LC neurons. The authors conclude that GPR139 modulates MOR control of neuronal excitability by a cell-autonomous mechanism. However it seems that these effects could also result from an action of GPR139 in non–MOR-expressing neurons within the slice. Future studies should determine which ion channels are involved in the reduced neuronal firing; they should also establish whether GPR139 modulates MOR-induced inhibition of neurotransmitter release and probe GPR139 function in other brain nuclei of critical importance for opioid addiction, including the nucleus accumbens and the ventral tegmental area (14), and implicated in OIRD, such as the pre-Bötzinger complex (15).


Mechanisms for reducing opioid side effects

GPR139 negatively regulates the µ-opioid receptor (MOR), reducing the cellular and behavioral responses that cause the harmful side effects of fentanyl and morphine. Three possible mechanisms include dimerization, inhibition of MOR trafficking and surface expression, and β-arrestin recruitment. GPR139 may have additional effects in signaling and in specific neuronal circuits.




Wang et al. investigated the importance of GPR139 in behavior. Mice in which Gpr139 was deleted showed an increased sensitivity to morphine-induced reward and analgesia. They also examined the potential of GPR139 as a drug target. When mice were given morphine and then the GPR139 agonist JNJ-63533054, Wang et al. observed a reversal of morphine-induced analgesia. When the authors examined the behavioral effects of JNJ-63533054 using a reward test (self-administering morphine), morphine was less rewarding. Although JNJ-63533054 reduced analgesia and has unknown effects on OIRD, this result is exciting because it suggests that GPR139 targeting could potentially be used in the treatment of opioid addiction. More work is needed to clarify the translational potential of GPR139, starting with determining the coexpression of GPR139 and MOR in regions mediating addiction and OIRD in the human brain. It remains possible that GPR139 regulates other GPCRs besides MOR and that GPR139 has important functions in other brain regions and beyond, both of which could result in detrimental side effects following administration of an agonist. Although these questions are unanswered, Wang et al. have pioneered the use of forward genetic approaches with C. elegans in the opioid field, a technique that could be rapidly used to identify testable drug targets to combat the ongoing opioid epidemic.




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