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A PTEN-Regulated Checkpoint Controls Surface Delivery of ? Opioid Receptors.

ABSTRACT: The ? opioid receptor (?R) is a promising alternate target for pain management because ?R agonists show decreased abuse potential compared with current opioid analgesics that target the ? opioid receptor. A critical limitation in developing ?R as an analgesic target, however, is that ?R agonists show relatively low efficacy in vivo, requiring the use of high doses that often cause adverse effects, such as convulsions. Here we tested whether intracellular retention of ?R in sensory neurons contributes to this low ?R agonist efficacy in vivo by limiting surface ?R expression. Using direct visualization of ?R trafficking and localization, we define a phosphatase and tensin homolog (PTEN)-regulated checkpoint that retains ?R in the Golgi and decreases surface delivery in rat and mice sensory neurons. PTEN inhibition releases ?R from this checkpoint and stimulates delivery of exogenous and endogenous ?R to the neuronal surface both in vitro and in vivo PTEN inhibition in vivo increases the percentage of TG neurons expressing ?R on the surface and allows efficient ?R-mediated antihyperalgesia in mice. Together, we define a critical role for PTEN in regulating the surface delivery and bioavailability of the ?R, explain the low efficacy of ?R agonists in vivo, and provide evidence that active ?R relocation is a viable strategy to increase ?R antinociception.SIGNIFICANCE STATEMENT Opioid analgesics, such as morphine, which target the ? opioid receptor (?R), have been the mainstay of pain management, but their use is highly limited by adverse effects and their variable efficacy in chronic pain. Identifying alternate analgesic targets is therefore of great significance. Although the ? opioid receptor (?R) is an attractive option, a critical limiting factor in developing ?R as a target has been the low efficacy of ?R agonists. Why ?R agonists show low efficacy is still under debate. This study provides mechanistic and functional data that intracellular localization of ?R in neurons is a key factor that contributes to low agonist efficacy, and presents a proof of mechanism that relocating ?R improves efficacy.

SUBMITTER: Shiwarski DJ

PROVIDER: S-EPMC5394893 | biostudies-literature | 2017 Apr

REPOSITORIES: biostudies-literature

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A PTEN-Regulated Checkpoint Controls Surface Delivery of δ Opioid Receptors.

Shiwarski Daniel J DJ Tipton Alycia A Giraldo Melissa D MD Schmidt Brigitte F BF Gold Michael S MS Pradhan Amynah A AA Puthenveedu Manojkumar A MA

The Journal of neuroscience : the official journal of the Society for Neuroscience 20170306 14

The δ opioid receptor (δR) is a promising alternate target for pain management because δR agonists show decreased abuse potential compared with current opioid analgesics that target the μ opioid receptor. A critical limitation in developing δR as an analgesic target, however, is that δR agonists show relatively low efficacy <i>in vivo</i>, requiring the use of high doses that often cause adverse effects, such as convulsions. Here we tested whether intracellular retention of δR in sensory neurons ...[more]

PMID: 28264976

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Project description:BackgroundGPCRs regulate a remarkable diversity of biological functions, and are thus often targeted for drug therapies. Stimulation of a GPCR by an extracellular ligand triggers receptor signaling via G proteins, and this process is highly regulated. Receptor activation is typically accompanied by desensitization of receptor signaling, a complex feedback regulatory process of which receptor internalization is postulated as a key event. The in vivo significance of GPCR internalization is poorly understood. In fact, the majority of studies have been performed in transfected cell systems, which do not adequately model physiological environments and the complexity of integrated responses observed in the whole animal.Methods and findingsIn this study, we used knock-in mice expressing functional fluorescent delta opioid receptors (DOR-eGFP) in place of the native receptor to correlate receptor localization in neurons with behavioral responses. We analyzed the pain-relieving effects of two delta receptor agonists with similar signaling potencies and efficacies, but distinct internalizing properties. An initial treatment with the high (SNC80) or low (AR-M100390) internalizing agonist equally reduced CFA-induced inflammatory pain. However, subsequent drug treatment produced highly distinct responses. Animals initially treated with SNC80 showed no analgesic response to a second dose of either delta receptor agonist. Concomitant receptor internalization and G-protein uncoupling were observed throughout the nervous system. This loss of function was temporary, since full DOR-eGFP receptor responses were restored 24 hours after SNC80 administration. In contrast, treatment with AR-M100390 resulted in retained analgesic response to a subsequent agonist injection, and ex vivo analysis showed that DOR-eGFP receptor remained G protein-coupled on the cell surface. Finally SNC80 but not AR-M100390 produced DOR-eGFP phosphorylation, suggesting that the two agonists produce distinct active receptor conformations in vivo which likely lead to differential receptor trafficking.ConclusionsTogether our data show that delta agonists retain full analgesic efficacy when receptors remain on the cell surface. In contrast, delta agonist-induced analgesia is abolished following receptor internalization, and complete behavioral desensitization is observed. Overall these results establish that, in the context of pain control, receptor localization fully controls receptor function in vivo. This finding has both fundamental and therapeutic implications for slow-recycling GPCRs.

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A PTEN-Regulated Checkpoint Controls Surface Delivery of ? Opioid Receptors.

Publications

A PTEN-Regulated Checkpoint Controls Surface Delivery of δ Opioid Receptors.

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