Phosphorylation of the delta-opioid receptor regulates its beta-arrestins selectivity and subsequent receptor internalization and adenylyl cyclase desensitization

ClickArr: a novel, high-throughput assay for evaluating β-arrestin isoform recruitment

Background: Modern methods for quantifying signaling bias at G protein-coupled receptors (GPCRs) rely on using a single β-arrestin isoform. However, it is increasingly appreciated that the two β-arrestin isoforms have unique roles, requiring the ability to assess β-arrestin isoform preference. Thus, methods are needed to efficiently screen the recruitment of both β-arrestin isoforms as they compete for a target GPCR in cells. Methods: We used molecular cloning to develop fusion proteins of the δ-opioid receptor (δOR), β-arrestin 1, and β-arrestin 2 to fragments of click beetle green and click beetle red luciferases. In this assay architecture, recruitment of either β-arrestin 1 or 2 to the δOR generates a spectrally distinct bioluminescent signal, allowing us to co-transfect all three constructs into cells prior to agonist challenge. Results: We demonstrate that our new assay, named "ClickArr," is a live-cell assay that simultaneously reports the recruitment of both β-arrestin isoforms as they compete for interaction with the δOR. We further find that the partial δOR agonist TAN67 has a significant efficacy bias for β-arrestin 2 over β-arrestin 1 when recruitment is normalized to the reference agonist leu-enkephalin. We confirm that ClickArr reports this bias when run either as a high-throughput endpoint or high-throughput kinetic assay, and cross-validate this result using the PathHunter assay, an orthogonal commercial assay for reporting β-arrestin recruitment to the δOR. Conclusion: Our results suggest that agonist:GPCR complexes can have relative β-arrestin isoform bias, a novel signaling bias that may potentially open up a new dimension for drug development.

Impact of T161, Y318 and S363 alanine mutations on regulation of the human delta-opioid receptor (hDOPr) induced by peptidic and alkaloid agonists

Previously, we showed a differential regulation of the human delta-opioid receptor (hDOPr) by etorphine and [D-Pen², D-Pen⁵] enkephalin (DPDPE). To understand the molecular basis of such differences, we introduced 3 alanine mutations at the residues T161. Y318 and S363. Both wild type (WT) and hDOPr mutants were expressed in HEK cells containing endogenous arrestins or CFP-tagged arrestin 3, then desensitization, internalization, recycling and phosphorylation were studied. In a context of endogenous arrestin expression, a major difference in DOPr desensitization was observed between agonists that was modified with the T161A mutation upon etorphine and with the S363A substitution upon DPDPE exposure. While both agonists induced a major receptor internalization, T161A and S363A impaired DOPr sequestration only for etorphine. However, similar level of S363 phosphorylation was measured between agonists. When CFP-tagged arrestin 3 was over-expressed, a similar profile of desensitization was measured for both agonists. In this context, all the 3 alanine mutations decreased etorphine-induced receptor desensitization. Using FRET, we showed similar interactions between WT hDOPr and arrestin 3 under DPDPE and etorphine stimulation which were delayed by both the Y318A and the S363A substitutions for etorphine. Finally, hDOPr recycling was qualitatively evaluated by microscopy and showed neither arrestin 3/hDOPr colocalization nor major impact of alanine mutations except for the S363A which impaired internalization and recycling for etorphine. The T161, Y318 and S363 residues of hDOPr could underlie the differential regulation promoted by DPDPE and etorphine.

Biased Opioid Ligands

Achieving effective pain management is one of the major challenges associated with modern day medicine. Opioids, such as morphine, have been the reference treatment for moderate to severe acute pain not excluding chronic pain modalities. Opioids act through the opioid receptors, the family of G-protein coupled receptors (GPCRs) that mediate pain relief through both the central and peripheral nervous systems. Four types of opioid receptors have been described, including the μ-opioid receptor (MOR), κ-opioid receptor (KOR), δ-opioid receptor (DOR), and the nociceptin opioid peptide receptor (NOP receptor). Despite the proven success of opioids in treating pain, there are still some inherent limitations. All clinically approved MOR analgesics are associated with adverse effects, which include tolerance, dependence, addiction, constipation, and respiratory depression. On the other hand, KOR selective analgesics have found limited clinical utility because they cause sedation, anxiety, dysphoria, and hallucinations. DOR agonists have also been investigated but they have a tendency to cause convulsions. Ligands targeting NOP receptor have been reported in the preclinical literature to be useful as spinal analgesics and as entities against substance abuse disorders while mixed MOR/NOP receptor agonists are useful as analgesics. Ultimately, the goal of opioid-related drug development has always been to design and synthesize derivatives that are equally or more potent than morphine but most importantly are devoid of the dangerous residual side effects and abuse potential. One proposed strategy is to take advantage of biased agonism, in which distinct downstream pathways can be activated by different molecules working through the exact same receptor. It has been proposed that ligands not recruiting β-arrestin 2 or showing a preference for activating a specific G-protein mediated signal transduction pathway will function as safer analgesic across all opioid subtypes. This review will focus on the design and the pharmacological outcomes of biased ligands at the opioid receptors, aiming at achieving functional selectivity.

Post-translational Modifications of Opioid Receptors

Post-translational modifications (PTMs) are key events in signal transduction since they affect protein function by regulating their abundance and/or activity. PTMs involve the covalent attachment of functional groups to specific amino acids. Since they tend to be generally reversible, PTMs serve as regulators of signal transduction pathways. G-protein-coupled receptors (GPCRs) are major signaling proteins that undergo multiple types of PTMs. In this Review, we focus on the opioid receptors, members of GPCR family A, and highlight recent advances in the field that have underscored the importance of PTMs in the functional regulation of these receptors. Since opioid receptor activity plays a central role in the development of tolerance and addiction to morphine and other drugs of abuse, understanding the molecular mechanisms regulating receptor activity is of fundamental importance.

Agonist-induced phosphorylation bar code and differential post-activation signaling of the delta opioid receptor revealed by phosphosite-specific antibodies

The δ-opioid receptor (DOP) is an attractive pharmacological target due to its potent analgesic, anxiolytic and anti-depressant activity in chronic pain models. However, some but not all selective DOP agonists also produce severe adverse effects such as seizures. Thus, the development of novel agonists requires a profound understanding of their effects on DOP phosphorylation, post-activation signaling and dephosphorylation. Here we show that agonist-induced DOP phosphorylation at threonine 361 (T361) and serine 363 (S363) proceeds with a temporal hierarchy, with S363 as primary site of phosphorylation. This phosphorylation is mediated by G protein-coupled receptor kinases 2 and 3 (GRK2/3) followed by DOP endocytosis and desensitization. DOP dephosphorylation occurs within minutes and is predominantly mediated by protein phosphatases (PP) 1α and 1β. A comparison of structurally diverse DOP agonists and clinically used opioids demonstrated high correlation between G protein-dependent signaling efficacies and receptor internalization. In vivo, DOP agonists induce receptor phosphorylation in a dose-dependent and agonist-selective manner that could be blocked by naltrexone in DOP-eGFP mice. Together, our studies provide novel tools and insights for ligand-activated DOP signaling in vitro and in vivo and suggest that DOP agonist efficacies may determine receptor post-activation signaling.

The Delta-Opioid Receptor; a Target for the Treatment of Pain

Nowadays, pain represents one of the most important societal burdens. Current treatments are, however, too often ineffective and/or accompanied by debilitating unwanted effects for patients dealing with chronic pain. Indeed, the prototypical opioid morphine, as many other strong analgesics, shows harmful unwanted effects including respiratory depression and constipation, and also produces tolerance, physical dependence, and addiction. The urgency to develop novel treatments against pain while minimizing adverse effects is therefore crucial. Over the years, the delta-opioid receptor (DOP) has emerged as a promising target for the development of new pain therapies. Indeed, targeting DOP to treat chronic pain represents a timely alternative to existing drugs, given the weak unwanted effects spectrum of DOP agonists. Here, we review the current knowledge supporting a role for DOP and its agonists for the treatment of pain. More specifically, we will focus on the cellular and subcellular localization of DOP in the nervous system. We will also discuss in further detail the molecular and cellular mechanisms involved in controlling the cellular trafficking of DOP, known to differ significantly from most G protein-coupled receptors. This review article will allow a better understanding of how DOP represents a promising target to develop new treatments for pain management as well as where we stand as of our ability to control its cellular trafficking and cell surface expression.

Agonist-dependent development of delta opioid receptor tolerance in the colon

The use of opioid analgesics is severely limited due to the development of intractable constipation, mediated through activation of mu opioid receptors (MOR) expressed by enteric neurons. The related delta opioid receptor (DOR) is an emerging therapeutic target for chronic pain, depression and anxiety. Whether DOR agonists also promote sustained inhibition of colonic transit is unknown. This study examined acute and chronic tolerance to SNC80 and ARM390, which were full and partial DOR agonists in neural pathways controlling colonic motility, respectively. Excitatory pathways developed acute and chronic tolerance to SNC80, whereas only chronic tolerance developed in inhibitory pathways. Both pathways remained functional after acute or chronic ARM390 exposure. Propagating colonic motor patterns were significantly reduced after acute or chronic SNC80 treatment, but not by ARM390 pre-treatment. These findings demonstrate that SNC80 has a prolonged inhibitory effect on propagating colonic motility. ARM390 had no effect on motor patterns and thus may have fewer gastrointestinal side-effects.

The δ-opioid receptor positive allosteric modulator BMS 986187 is a G-protein-biased allosteric agonist

BACKGROUND AND PURPOSE

The δ-opioid receptor is an emerging target for the management of chronic pain and depression. Biased signalling, the preferential activation of one signalling pathway over another downstream of δ-receptors, may generate better therapeutic profiles. BMS 986187 is a positive allosteric modulator of δ-receptors. Here, we ask if BMS 986187 can directly activate the receptor from an allosteric site, without an orthosteric ligand, and if a signalling bias is generated.

EXPERIMENTAL APPROACH

We used several clonal cell lines expressing δ-receptors, to assess effects of BMS 986187 on events downstream of δ-receptors by measuring G-protein activation, β-arrestin 2 recruitment, receptor phosphorylation, loss of surface receptor expression, ERK1/ERK2 phosphorylation, and receptor desensitization.

KEY RESULTS

BMS 986187 is a G protein biased allosteric agonist, relative to β-arrestin 2 recruitment. Despite showing direct and potent G protein activation, BMS 986187 has a low potency to recruit β-arrestin 2. This appears to reflect the inability of BMS 986187 to elicit any significant receptor phosphorylation, consistent with low receptor internalization and a slower onset of desensitization, compared with the full agonist SNC80.

CONCLUSIONS AND IMPLICATIONS

This is the first evidence of biased agonism mediated through direct binding to an allosteric site on an opioid receptor, without a ligand at the orthosteric site. Our data suggest that agonists targeting δ-receptors, or indeed any GPCR, through allosteric sites may be a novel way to promote signalling bias and thereby potentially produce a more specific pharmacology than can be observed by activation via the orthosteric site.

Optical approaches for single-cell and subcellular analysis of GPCR-G protein signaling

G protein-coupled receptors (GPCRs), G proteins, and their signaling associates are major signal transducers that control the majority of cellular signaling and regulate key biological functions including immune, neurological, cardiovascular, and metabolic processes. These pathways are targeted by over one-third of drugs on the market; however, the current understanding of their function is limited and primarily derived from cell-destructive approaches providing an ensemble of static, multi-cell information about the status and composition of molecules. Spatiotemporal behavior of molecules involved is crucial to understanding in vivo cell behaviors both in health and disease, and the advent of genetically encoded fluorescence proteins and small fluorophore-based biosensors has facilitated the mapping of dynamic signaling in cells with subcellular acuity. Since we and others have developed optogenetic methods to regulate GPCR-G protein signaling in single cells and subcellular regions using dedicated wavelengths, the desire to develop and adopt optogenetically amenable assays to measure signaling has motivated us to take a broader look at the available optical tools and approaches compatible with measuring single-cell and subcellular GPCR-G protein signaling. Here we review such key optical approaches enabling the examination of GPCR, G protein, secondary messenger, and downstream molecules such as kinase and lipid signaling in living cells. The methods reviewed employ both fluorescence and bioluminescence detection. We not only further elaborate the underlying principles of these sensors but also discuss the experimental criteria and limitations to be considered during their use in single-cell and subcellular signal mapping.

Tolerance to high-internalizing δ opioid receptor agonist is critically mediated by arrestin 2

BACKGROUND AND PURPOSE

Opioid δ receptor agonists are potent antihyperalgesics in chronic pain models, but tolerance develops after prolonged use. Previous evidence indicates that distinct forms of tolerance occur depending on the internalization properties of δ receptor agonists. As arrestins are important in receptor internalization, we investigated the role of arrestin 2 (β-arrestin 1) in mediating the development of tolerance induced by high- and low-internalizing δ receptor agonists.

EXPERIMENTAL APPROACH

We evaluated the effect of two δ receptor agonists with similar analgesic potencies, but either high-(SNC80) or low-(ARM390) internalization properties in wild-type (WT) and arrestin 2 knockout (KO) mice. We compared tolerance to the antihyperalgesic effects of these compounds in a model of inflammatory pain. We also examined tolerance to the convulsant effect of SNC80. Furthermore, effect of chronic treatment with SNC80 on δ agonist-stimulated [³⁵ S]-GTPγS binding was determined in WT and KO mice.

KEY RESULTS

Arrestin 2 KO resulted in increased drug potency, duration of action and decreased acute tolerance to the antihyperalgesic effects of SNC80. In contrast, ARM390 produced similar effects in both WT and KO animals. Following chronic treatment, we found a marked decrease in the extent of tolerance to SNC80-induced antihyperalgesia and convulsions in arrestin 2 KO mice. Accordingly, δ receptors remained functionally coupled to G proteins in arrestin 2 KO mice chronically treated with SNC80.

CONCLUSIONS AND IMPLICATIONS

Overall, these results suggest that δ receptor agonists interact with arrestins in a ligand-specific manner, and tolerance to high- but not low-internalizing agonists are preferentially regulated by arrestin 2.

C-terminus of OX2R significantly affects downstream signaling pathways

The human orexin 2 receptor (OX2R) is a G-protein‑coupled receptor (GPCR) that has been implicated in a number of diverse physiological functions. Recent studies have identified a number of functions of the C‑termini of GPCRs. However, the importance of the OX2R C‑terminus in regulating signaling and surface expression remains unclear. In the present study, the function of the OX2R C‑terminus was investigated using three C‑terminal mutants, which were truncated at residues 368, 384 and 414, respectively, and the wild‑type control, which expressed the full‑length OX2R. HEK‑293 cells were transfected with the mutated and control OX2R constructs. ELISA, western blot analysis and calcium assays were used to investigate the effects of the mutations on OX2R function. The present results demonstrated that residues 385‑414 and 415‑444 exhibited a cumulative effect on the surface expression of OX2R. Residues 369‑384 exhibited a significant influence on inositol phosphate production and extracellular signal‑regulated kinase 1/2 phosphorylation. Residues 385‑414 significantly influenced agonist‑induced internalization, whereas residues 369‑384 and 385‑414 significantly influenced Ca2+ release. The results of the present study suggest that the C‑terminus of OX2R is important for its role in various physiological and pathological processes, and may therefore be associated with such disorders as depression and anorexia.

Molecular Pharmacology of δ-Opioid Receptors

Opioids are among the most effective analgesics available and are the first choice in the treatment of acute severe pain. However, partial efficacy, a tendency to produce tolerance, and a host of ill-tolerated side effects make clinically available opioids less effective in the management of chronic pain syndromes. Given that most therapeutic opioids produce their actions via µ-opioid receptors (MOPrs), other targets are constantly being explored, among which δ-opioid receptors (DOPrs) are being increasingly considered as promising alternatives. This review addresses DOPrs from the perspective of cellular and molecular determinants of their pharmacological diversity. Thus, DOPr ligands are examined in terms of structural and functional variety, DOPrs' capacity to engage a multiplicity of canonical and noncanonical G protein-dependent responses is surveyed, and evidence supporting ligand-specific signaling and regulation is analyzed. Pharmacological DOPr subtypes are examined in light of the ability of DOPr to organize into multimeric arrays and to adopt multiple active conformations as well as differences in ligand kinetics. Current knowledge on DOPr targeting to the membrane is examined as a means of understanding how these receptors are especially active in chronic pain management. Insight into cellular and molecular mechanisms of pharmacological diversity should guide the rational design of more effective, longer-lasting, and better-tolerated opioid analgesics for chronic pain management.

Agonist-Specific Recruitment of Arrestin Isoforms Differentially Modify Delta Opioid Receptor Function

Ligand-specific recruitment of arrestins facilitates functional selectivity of G-protein-coupled receptor signaling. Here, we describe agonist-selective recruitment of different arrestin isoforms to the delta opioid receptor in mice. A high-internalizing delta opioid receptor agonist (SNC80) preferentially recruited arrestin 2 and, in arrestin 2 knock-outs (KOs), we observed a significant increase in the potency of SNC80 to inhibit mechanical hyperalgesia and decreased acute tolerance. In contrast, the low-internalizing delta agonists (ARM390, JNJ20788560) preferentially recruited arrestin 3 with unaltered behavioral effects in arrestin 2 KOs. Surprisingly, arrestin 3 KO revealed an acute tolerance to these low-internalizing agonists, an effect never observed in wild-type animals. Furthermore, we examined delta opioid receptor-Ca(2+)channel coupling in dorsal root ganglia desensitized by ARM390 and the rate of resensitization was correspondingly decreased in arrestin 3 KOs. Live-cell imaging in HEK293 cells revealed that delta opioid receptors are in pre-engaged complexes with arrestin 3 at the cell membrane and that ARM390 strengthens this membrane interaction. The disruption of these complexes in arrestin 3 KOs likely accounts for the altered responses to low-internalizing agonists. Together, our results show agonist-selective recruitment of arrestin isoforms and reveal a novel endogenous role of arrestin 3 as a facilitator of resensitization and an inhibitor of tolerance mechanisms.

SIGNIFICANCE STATEMENT

Agonists that bind to the same receptor can produce highly distinct signaling events and arrestins are a major mediator of this ligand bias. Here, we demonstrate that delta opioid receptor agonists differentially recruit arrestin isoforms. We found that the high-internalizing agonist SNC80 preferentially recruits arrestin 2 and knock-out (KO) of this protein results in increased efficacy of SNC80. In contrast, low-internalizing agonists (ARM390 and JNJ20788560) preferentially recruit arrestin 3 and, surprisingly, KO of arrestin 3 produces acute tolerance and impaired receptor resensitization to these agonists. Arrestin 3 is in pre-engaged complexes with the delta opioid receptor at the cell membrane and low-internalizing agonists promote this interaction. This study reveals a novel role for arrestin 3 as a facilitator of receptor resensitization.

The delta opioid receptor tool box

In recent years, the delta opioid receptor has attracted increasing interest as a target for the treatment of chronic pain and emotional disorders. Due to their therapeutic potential, numerous tools have been developed to study the delta opioid receptor from both a molecular and a functional perspective. This review summarizes the most commonly available tools, with an emphasis on their use and limitations. Here, we describe (1) the cell-based assays used to study the delta opioid receptor. (2) The features of several delta opioid receptor ligands, including peptide and non-peptide drugs. (3) The existing approaches to detect delta opioid receptors in fixed tissue, and debates that surround these techniques. (4) Behavioral assays used to study the in vivo effects of delta opioid receptor agonists; including locomotor stimulation and convulsions that are induced by some ligands, but not others. (5) The characterization of genetically modified mice used specifically to study the delta opioid receptor. Overall, this review aims to provide a guideline for the use of these tools with the final goal of increasing our understanding of delta opioid receptor physiology.

Biased signalling: the instinctive skill of the cell in the selection of appropriate signalling pathways

GPCRs (G-protein-coupled receptors) are members of a family of proteins which are generally regarded as the largest group of therapeutic drug targets. Ligands of GPCRs do not usually activate all cellular signalling pathways linked to a particular seven-transmembrane receptor in a uniform manner. The fundamental idea behind this concept is that each ligand has its own ability, while interacting with the receptor, to activate different signalling pathways (or a particular set of signalling pathways) and it is this concept which is known as biased signalling. The importance of biased signalling is that it may selectively activate biological responses to favour therapeutically beneficial signalling pathways and to avoid adverse effects. There are two levels of biased signalling. First, bias can arise from the ability of GPCRs to couple to a subset of the available G-protein subtypes: Gαs, Gαq/11, Gαi/o or Gα12/13. These subtypes produce the diverse effects of GPCRs by targeting different effectors. Secondly, biased GPCRs may differentially activate G-proteins or β-arrestins. β-Arrestins are ubiquitously expressed and function to terminate or inhibit classic G-protein signalling and initiate distinct β-arrestin-mediated signalling processes. The interplay of G-protein and β-arrestin signalling largely determines the cellular consequences of the administration of GPCR-targeted drugs. In the present review, we highlight the particular functionalities of biased signalling and discuss its biological effects subsequent to GPCR activation. We consider that biased signalling is potentially allowing a choice between signalling through ‘beneficial’ pathways and the avoidance of ‘harmful’ ones.

In vivo techniques to investigate the internalization profile of opioid receptors

G-protein-coupled receptors (GPCRs) regulate a remarkable diversity of biological functions, and are thus often targeted for drug therapies. Receptor internalization is commonly observed following agonist binding and activation. Receptor trafficking events have been well characterized in cell systems, but the in vivo significance of GPCR internalization is still poorly understood. To address this issue, we have developed an innovative knock-in mouse model, where an opioid receptor is directly visible in vivo. These knockin mice express functional fluorescent delta opioid receptors (DOR-eGFP) in place of the endogenous receptor, and these receptors are expressed at physiological levels within their native environment. DOR-eGFP mice have proven to be an extraordinary tool in studying receptor neuroanatomy, real-time receptor trafficking in live neurons, and in vivo receptor internalization. We have used this animal model to determine the relationship between receptor trafficking in neurons and receptor function at a behavioral level. Here, we describe in detail the construction and characterization of this knockin mouse. We also outline how to use these mice to examine the behavioral consequences of agonist-specific trafficking at the delta opioid receptor. These techniques are potentially applicable to any GPCR, and highlight the powerful nature of this imaging tool.

Role of opioid receptor heterodimerization in pain modulation and tolerance development

Protein to protein interactions leading to homo/heteromerization of receptor is well documented in literature. These interactions leading to dimeric/oligomers formation of receptors are known to modulate their function, particularly in case of G-protein coupled receptors. The opioid receptor heteromers having changed pharmacological properties than the constituent protomers provides preferences for novel drug targets that could lead to potential analgesic activity devoid of tolerance and physical dependence. Heterodimerization of opioid receptors appears to generate novel binding properties with improved specificity and lack of side effects. Further the molecules which can interact simultaneously to both the protomers of the heteromer, or to both the binding sites (orthosteric and allosteric) of a receptor protein could be potential therapeutic molecules. This review highlights the recent advancements in exploring the plausible role of heteromerization of opioid receptors in induction of tolerance free antinociception.

Opioid receptor desensitization: mechanisms and its link to tolerance

Opioid receptors (OR) are part of the class A of G-protein coupled receptors and the target of the opiates, the most powerful analgesic molecules used in clinic. During a protracted use, a tolerance to analgesic effect develops resulting in a reduction of the effectiveness. So understanding mechanisms of tolerance is a great challenge and may help to find new strategies to tackle this side effect. This review will summarize receptor-related mechanisms that could underlie tolerance especially receptor desensitization. We will focus on the latest data obtained on molecular mechanisms involved in opioid receptor desensitization: phosphorylation, receptor uncoupling, internalization, and post-endocytic fate of the receptor.

β-arrestin-2-biased agonism of delta opioid receptors sensitizes transient receptor potential vanilloid type 1 (TRPV1) in primary sensory neurons

Despite advances in understanding the signaling mechanisms involved in the development and maintenance of chronic pain, the pharmacologic treatment of chronic pain has seen little advancement. Agonists at the mu opioid receptor (MOPr) continue to be vital in the treatment of many forms of chronic pain, but side-effects limit their clinical utility and range from relatively mild, such as constipation, to major, such as addiction and dependence. Additionally, chronic activation of MOPr results in pain hypersensitivity known as opioid-induced hyperalgesia (OIH), and we have shown recently that recruitment of β-arrestin2 to MOPr, away from transient potential vanilloid eceptor type 1 (TRPV1) in primary sensory neurons contributes to this phenomenon. The delta opioid receptor (DOPr) has become a promising target for the treatment of chronic pain, but little is known about the effects of chronic activation of DOPr on nociceptor sensitivity and OIH. Here we report that chronic activation of DOPr by the DOPr-selective agonist, SNC80, results in the sensitization of TRPV1 and behavioral signs of OIH via β-arrestin2 recruitment to DOPr and away from TRPV1. Conversely, chronic treatment with ARM390, a DOPr-selective agonist that does not recruit β-arrestin2, neither sensitized TRPV1 nor produced OIH. Interestingly, the effect of SNC80 to sensitize TRPV1 is species-dependent, as rats developed OIH but mice did not. Taken together, the reported data identify a novel side-effect of chronic administration of β-arrestin2-biased DOPr agonists and highlight the importance of potential species-specific effects of DOPr agonists.

Ligand- and cell-dependent determinants of internalization and cAMP modulation by delta opioid receptor (DOR) agonists

Signaling bias refers to G protein-coupled receptor ligand ability to preferentially activate one type of signal over another. Bias to evoke signaling as opposed to sequestration has been proposed as a predictor of opioid ligand potential for generating tolerance. Here we measured whether delta opioid receptor agonists preferentially inhibited cyclase activity over internalization in HEK cells. Efficacy (τ) and affinity (KA) values were estimated from functional data and bias was calculated from efficiency coefficients (log τ/KA). This approach better represented the data as compared to alternative methods that estimate bias exclusively from τ values. Log (τ/KA) coefficients indicated that SNC-80 and UFP-512 promoted cyclase inhibition more efficiently than DOR internalization as compared to DPDPE (bias factor for SNC-80: 50 and for UFP-512: 132). Molecular determinants of internalization were different in HEK293 cells and neurons with βarrs contributing to internalization in both cell types, while PKC and GRK2 activities were only involved in neurons. Rank orders of ligand ability to engage different internalization mechanisms in neurons were compared to rank order of E max values for cyclase assays in HEK cells. Comparison revealed a significant reversal in rank order for cyclase E max values and βarr-dependent internalization in neurons, indicating that these responses were ligand-specific. Despite this evidence, and because kinases involved in internalization were not the same across cellular backgrounds, it is not possible to assert if the magnitude and nature of bias revealed by rank orders of maximal responses is the same as the one measured in HEK cells.

Select G-protein-coupled receptors modulate agonist-induced signaling via a ROCK, LIMK, and β-arrestin 1 pathway

G-protein-coupled receptors (GPCRs) are typically present in a basal, inactive state but, when bound to an agonist, activate downstream signaling cascades. In studying arrestin regulation of opioid receptors in dorsal root ganglia (DRG) neurons, we find that agonists of delta opioid receptors (δORs) activate cofilin through Rho-associated coiled-coil-containing protein kinase (ROCK), LIM domain kinase (LIMK), and β-arrestin 1 (β-arr1) to regulate actin polymerization. This controls receptor function, as assessed by agonist-induced inhibition of voltage-dependent Ca(2+) channels in DRGs. Agonists of opioid-receptor-like receptors (ORL1) similarly influence the function of this receptor through ROCK, LIMK, and β-arr1. Functional evidence of this cascade was demonstrated in vivo, where the behavioral effects of δOR or ORL1 agonists were enhanced in the absence of β-arr1 or prevented by inhibiting ROCK. This pathway allows δOR and ORL1 agonists to rapidly regulate receptor function.

Chronic inflammatory injury results in increased coupling of delta opioid receptors to voltage-gated Ca2+ channels

Background

Opioid receptors regulate a diverse array of physiological functions. Mu opioid receptor agonists are well-known analgesics for treating acute pain. In contrast, animal models suggest that chronic pain is more effectively relieved by delta opioid receptor agonists. A number of studies have shown that chronic pain results in increased function of delta opioid receptors. This is proposed to result from enhanced trafficking of the delta opioid receptor to the cell membrane induced by persistent tissue injury. However, recent studies have questioned this mechanism, which has resulted in some uncertainty as to whether delta opioid receptors are indeed upregulated in chronic pain states. To clarify this question, we have examined the effect of chronic inflammatory pain over time using both an ex vivo measure of delta function: receptor-Ca²⁺ channel coupling, and an in vivo measure; the relief of chronic pain by a delta opioid receptor agonist. In addition, as beta-arrestin 2 can regulate delta opioid receptor trafficking and signaling, we have further examined whether deleting this scaffolding and signal transduction molecule alters delta opioid receptor function.

Results

We used the Complete Freund’s Adjuvant model of inflammatory pain, and examined the effectiveness of the delta agonist, SNC80, to both inhibit Ca²⁺ channels in primary afferent neurons and to attenuate mechanical allodynia. In naïve beta-arrestin 2 wildtype and knockout mice, SNC80 neither significantly inhibited voltage-dependent Ca²⁺ currents nor produced antinociception. However, following inflammatory pain, both measures showed a significant and long-lasting enhancement of delta opioid receptor function that persisted for up to 14 days post-injury regardless of genotype. Furthermore, although this pain model did not alter Ca²⁺ current density, the contribution of N-type Ca²⁺ channels to the total current appeared to be regulated by the presence of beta-arrestin 2.

Conclusions

Our results indicate that there is an upregulation of delta opioid receptor function following chronic pain. This gain of function is reflected in the increased efficacy of a delta agonist in both behavioral and electrophysiological measures. Overall, this work confirms that delta opioid receptors can be enhanced following tissue injury associated with chronic pain.

Arrestins: role in the desensitization, sequestration, and vesicular trafficking of G protein-coupled receptors

Over the years, β-arrestins have emerged as multifunctional molecular scaffolding proteins regulating almost every imaginable G protein-coupled receptor (GPCR) function. Originally discovered as GPCR-desensitizing molecules, they have been shown to also serve as important regulators of GPCR signaling, sequestration, and vesicular trafficking. This broad functional role implicates β-arrestins as key regulatory proteins for cellular function. Hence, this chapter summarizes the current understanding of the β-arrestin family's unique ability to control the kinetics as well as the extent of GPCR activity at the level of desensitization, sequestration, and subsequent intracellular trafficking.

δ-Opioid mechanisms for ADL5747 and ADL5859 effects in mice: analgesia, locomotion, and receptor internalization

N,N-diethyl-4-(5-hydroxyspiro[chromene-2,4'-piperidine]-4-yl) benzamide (ADL5859) and N,N-diethyl-3-hydroxy-4-(spiro[chromene-2,4'-piperidine]-4-yl)benzamide (ADL5747) are novel δ-opioid agonists that show good oral bioavailability and analgesic and antidepressive effects in the rat and represent potential drugs for chronic pain treatment. Here, we used genetic approaches to investigate molecular mechanisms underlying their analgesic effects in the mouse. We tested analgesic effects of ADL5859 and ADL5747 in mice by using mechanical sensitivity measures in both complete Freund's adjuvant and sciatic nerve ligation pain models. We examined their analgesic effects in δ-opioid receptor constitutive knockout (KO) mice and mice with a conditional deletion of δ-receptor in peripheral voltage-gated sodium channel (Nav)1.8-expressing neurons (cKO mice). Both ADL5859 and ADL5747, and the prototypical δ agonist 4-[(R)-[(2S,5R)-4-allyl-2,5-dimethyl-piperazin-1-yl]-(3-methoxyphenyl)methyl]-N,N-diethyl-benzamide (SNC80) as a control, significantly reduced inflammatory and neuropathic pain. The antiallodynic effects of all three δ-opioid agonists were abolished in constitutive δ-receptor KO mice and strongly diminished in δ-receptor cKO mice. We also measured two other well described effects of δ agonists, increase in locomotor activity and agonist-induced receptor internalization by using knock-in mice expressing enhanced green fluorescence protein-tagged δ receptors. In contrast to SNC80, ADL5859 and ADL5747 did not induce either hyperlocomotion or receptor internalization in vivo. In conclusion, both ADL5859 and ADL5747 showed efficient pain-reducing properties in the two models of chronic pain. Their effects were mediated by δ-opioid receptors, with a main contribution of receptors expressed on peripheral Nav1.8-positive neurons. The lack of in vivo receptor internalization and locomotor activation, typically induced by SNC80, suggests agonist-biased activity at the receptor for the two drugs.

Differential association of receptor-Gβγ complexes with β-arrestin2 determines recycling bias and potential for tolerance of δ opioid receptor agonists

Opioid tendency to generate analgesic tolerance has been previously linked to biased internalization. Here, we assessed an alternative possibility; whether tolerance of delta opioid receptor agonists (DORs) could be related to agonist-specific recycling. A first series of experiments revealed that DOR internalization by DPDPE and SNC-80 was similar, but only DPDPE induced recycling. We then established that the non-recycling agonist SNC-80 generated acute analgesic tolerance that was absent in mice treated with DPDPE. Furthermore, both agonists stabilized different conformations, whose distinct interaction with Gβγ subunits led to different modalities of β-arrestin2 (βarr2) recruitment. In particular, bioluminescence resonance energy transfer (BRET) assays revealed that sustained activation by SNC-80 drew the receptor C terminus in close proximity of the N-terminal domain of Gγ2, causing βarr2 to interact with receptors and Gβγ subunits. DPDPE moved the receptor C-tail away from the Gβγ dimer, resulting in βarr2 recruitment to the receptor but not in the vicinity of Gγ2. These differences were associated with stable DOR-βarr2 association, poor recycling, and marked desensitization following exposure to SNC-80, while DPDPE promoted transient receptor interaction with βarr2 and effective recycling, which conferred protection from desensitization. Together, these data indicate that DORs may adopt ligand-specific conformations whose distinct recycling properties determine the extent of desensitization and are predictive of analgesic tolerance. Based on these findings, we propose that the development of functionally selective DOR ligands that favor recycling could constitute a valid strategy for the production of longer acting opioid analgesics.

Fluorescence/bioluminescence resonance energy transfer techniques to study G-protein-coupled receptor activation and signaling

Fluorescence and bioluminescence resonance energy transfer (FRET and BRET) techniques allow the sensitive monitoring of distances between two labels at the nanometer scale. Depending on the placement of the labels, this permits the analysis of conformational changes within a single protein (for example of a receptor) or the monitoring of protein-protein interactions (for example, between receptors and G-protein subunits). Over the past decade, numerous such techniques have been developed to monitor the activation and signaling of G-protein-coupled receptors (GPCRs) in both the purified, reconstituted state and in intact cells. These techniques span the entire spectrum from ligand binding to the receptors down to intracellular second messengers. They allow the determination and the visualization of signaling processes with high temporal and spatial resolution. With these techniques, it has been demonstrated that GPCR signals may show spatial and temporal patterning. In particular, evidence has been provided for spatial compartmentalization of GPCRs and their signals in intact cells and for distinct physiological consequences of such spatial patterning. We review here the FRET and BRET technologies that have been developed for G-protein-coupled receptors and their signaling proteins (G-proteins, effectors) and the concepts that result from such experiments.

ßarrestin1-biased agonism at human δ-opioid receptor by peptidic and alkaloid ligands

We have previously reported on the differential regulation of the human δ-opioid receptor (hDOR) by alkaloid (etorphine) and peptidic (DPDPE and deltorphin I) ligands, in terms of both receptor desensitization and post-endocytic sorting. Since ßarrestins are well known to regulate G protein-coupled receptors (GPCRs) signaling and trafficking, we therefore investigated the role of ßarrestin1 (the only isoform expressed in our cellular model) in the context of the hDOR. We established clonal cell lines of SK-N-BE cells over-expressing ßarrestin1, its dominant negative mutant (ßarrestin1(319-418)), and shRNA directed against endogenous ßarrestin1. Interestingly, both binding and confocal microscopy approaches demonstrated that ßarrestin1 is required for hDOR endocytosis only when activated by etorphine. Conversely, functional experiments revealed that ßarrestin1 is exclusively involved in hDOR desensitization promoted by the peptides. Taken together, these results provide substantial evidence for a ßarrestin1-biased agonism at hDOR, where ßarrestin1 is differentially involved during receptor desensitization and endocytosis depending on the ligand.

Regulation of opioid receptor signalling: implications for the development of analgesic tolerance

Opiate drugs are the most effective analgesics available but their clinical use is restricted by severe side effects. Some of these undesired actions appear after repeated administration and are related to adaptive changes directed at counteracting the consequences of sustained opioid receptor activation. Here we will discuss adaptations that contribute to the development of tolerance. The focus of the first part of the review is set on molecular mechanisms involved in the regulation of opioid receptor signalling in heterologous expression systems and neurons. In the second part we assess how adaptations that take place in vivo may contribute to analgesic tolerance developed during repeated opioid administration.

Serine 363 of the {delta}-opioid receptor is crucial for adopting distinct pathways to activate ERK1/2 in response to stimulation with different ligands

Distinct opioid receptor agonists have been proved to induce differential patterns of ERK activation, but the underlying mechanisms remain unclear. Here, we report that Ser363 in the δ-opioid receptor (δOR) determines the different abilities of the δOR agonists DPDPE and TIPP to activate ERK by G-protein- or β-arrestin-dependent pathways. Although both DPDPE and TIPP activated ERK1/2, they showed different temporal, spatial and desensitization patterns of ERK activation. We show that that DPDPE employed G protein as the primary mediator to activate the ERK cascade in an Src-dependent manner, whereas TIPP mainly adopted a β-arrestin1/2-mediated pathway. Moreover, we found that DPDPE gained the capacity to adopt the β-arrestin1/2-mediated pathway upon Ser363 mutation, accompanied by the same pattern of ERK activation as that induced by TIPP. Additionally, we found that TIPP- but not DPDPE-activated ERK could phosphorylate G-protein-coupled receptor kinase-2 and β-arrestin1. However, such functional differences of ERK disappeared with the mutation of Ser363. Therefore, the present study reveals a crucial role for Ser363 in agonist-specific regulation of ERK activation patterns and functions.

Adenosine A(1) receptor agonist N(6)-cyclohexyl-adenosine induced phosphorylation of delta opioid receptor and desensitization of its signaling

AIM

To define the effect of adenosine A(1) receptor (A(1)R) on delta opioid receptor (DOR)-mediated signal transduction.

METHODS

CHO cells stably expressing HA-tagged A(1)R and DOR-CFP fusion protein were used. The localization of receptors was observed using confocal microscope. DOR-mediated inhibition of adenylyl cyclase was measured using cyclic AMP assay. Western blots were employed to detect the phosphorylation of Akt and the DOR. The effect of A(1)R agonist N(6)-cyclohexyladenosine (CHA) on DOR down-regulation was assessed using radioligand binding assay.

RESULTS

CHA 1 micromol/L time-dependently attenuated DOR agonist [D-Pen(2,5)]enkephalin (DPDPE)-induced inhibition of intracellular cAMP accumulation with a t(1/2)=2.56 (2.09-3.31) h. Pretreatment with 1 micromol/L CHA for 24 h caused a right shift of the dose-response curve of DPDPE-mediated inhibition of cAMP accumulation, with a significant increase in EC(50) but no change in E(max). Pretreatment with 1 micromol/L CHA for 1 h also induced a significant attenuation of DPDPE-stimulated phosphorylation of Akt. Moreover, CHA time-dependently phosphorylated DOR (Ser363), and this effect was inhibited by A(1)R antagonist 1,3-Dipropyl-8-cyclopentylxanthine (DPCPX) but not by DOR antagonist naloxone. However, CHA failed to produce the down-regulation of DOR, as neither receptor affinity (K(d)) nor receptor density (B(max)) of DOR showed significant change after chronic CHA exposure.

CONCLUSION

Activation of A(1)R by its agonist caused heterologous desensitization of DOR-mediated inhibition of intracellular cAMP accumulation and phosphorylation of Akt. Activation of A(1)R by its agonist also induced heterologous phosphorylation but not down-regulation of DOR.

Opioid-receptor-heteromer-specific trafficking and pharmacology

Homomerization and heteromerization of 7 transmembrane spanning (7TM)/G-protein-coupled receptors (GPCRs) have been an important field of study. Whereas initial studies were performed in artificial cell systems, recent publications are shifting the focus to the in vivo relevance of heteromerization. This is especially apparent for the field of opioid receptors. Drugs have been identified that selectively target opioid heteromers of the delta-opioid receptor with the kappa and the mu-opioid receptors that influence nociception and ethanol consumption, respectively. In addition, in several cases, the specific physiological response produced by the heteromer may be directly attributed to a difference in receptor trafficking properties of the heteromers compared with their homomeric counterparts. This review attempts to highlight some of the latest developments with regard to opioid receptor heteromer trafficking and pharmacology.

Recombinant C3adesArg/acylation stimulating protein (ASP) is highly bioactive: a critical evaluation of C5L2 binding and 3T3-L1 adipocyte activation

C5L2 is a recently identified receptor for C5a/C5adesArg, C3a and C3adesArg (ASP). C5a/C5adesArg bind with high affinity, with no identified activation. By contrast, some studies demonstrate C3a/ASP binding/activation to C5L2; others do not. Our aim is to critically evaluate ASP/C3adesArg-C5L2 binding and bioactivity. Cell-associated fluorescent-ASP (Fl-ASP) binding to C5L2 increased from transiently transfected<stably transfected<Fl-ASP-sorted C5L2-HEK for both human C5L2 and mouse C5L2. Transfected C5L2-CHO cells had similar results. Endogenous C5L2 expression increased from 3T3-L1 preadipocytes<3T3-L1 adipocytes<primary mouse adipocytes. Non-transfected cells+/-Fl-ASP demonstrated background fluorescence only. In adherent C5L2-HEK (Fl-ASP sorted) and 3T3-L1 cells, blocking with 10% fetal calf serum, protamine sulfate or ovalbumin prevented (125)I-ASP non-specific binding (NSB, no cells), while albumin increased NSB. Binding to non-transfected HEK was comparable to NSB. Optimal specific binding was obtained at 20 degrees C (vs. 4 degrees C) in PBS or serum-free medium with K(d) 83.7+/-23.7 nM (C5L2-HEK), 66+/-15 nM (C5L2-CHO) and 76+/-14.3 nM (3T3-L1 preadipocytes); (125)I-C5a binding had greater affinity. Fl-ASP-C5L2 binding was comparable and concentration dependent (K(d) 31 nM (direct binding) and IC(50) 35 nM (competition binding) regardless of conditions). Recombinant ASP (rASP) produced in modified Escherichia coli Origami (DE3) (allowing folding and disulphide bridge formation), purified under non-denaturing conditions demonstrated 10x greater bioactivity vs. proteolytically derived plasma ASP for triglyceride synthesis and fatty acid uptake in 3T3-L1 adipocytes and preadipocytes while adipose tissue from C5L2 KO mice was non-responsive. rASP stimulation of adipocyte BODIPY-fatty acid uptake demonstrated EC(50) 115+/-93 nM and maximal stimulation of 413+/-33%, p<0.001. ASP binding has distinct characteristics that lead to C5L2 activation and increased bioactivity.

Modulation of opioid receptor function by protein-protein interactions

Opioid receptors, MORP, DORP and KORP, belong to the family A of G protein coupled receptors (GPCR), and have been found to modulate a large number of physiological functions, including mood, stress, appetite, nociception and immune responses. Exogenously applied opioid alkaloids produce analgesia, hedonia and addiction. Addiction is linked to alterations in function and responsiveness of all three opioid receptors in the brain. Over the last few years, a large number of studies identified protein-protein interactions that play an essential role in opioid receptor function and responsiveness. Here, we summarize interactions shown to affect receptor biogenesis and trafficking, as well as those affecting signal transduction events following receptor activation. This article also examines protein interactions modulating the rate of receptor endocytosis and degradation, events that play a major role in opiate analgesia. Like several other GPCRs, opioid receptors may form homo or heterodimers. The last part of this review summarizes recent knowledge on proteins known to affect opioid receptor dimerization.

Endogenous opiates and behavior: 2007

This paper is the thirtieth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2007 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior, and the roles of these opioid peptides and receptors in pain and analgesia; stress and social status; tolerance and dependence; learning and memory; eating and drinking; alcohol and drugs of abuse; sexual activity and hormones, pregnancy, development and endocrinology; mental illness and mood; seizures and neurologic disorders; electrical-related activity and neurophysiology; general activity and locomotion; gastrointestinal, renal and hepatic functions; cardiovascular responses; respiration and thermoregulation; and immunological responses.

Direct comparison of bioluminescence-based resonance energy transfer methods for monitoring of proteolytic cleavage

Bioluminescence resonance energy transfer (BRET) is a powerful tool for the study of protein-protein interactions and conformational changes within proteins. Two common implementations of BRET are BRET(1) with Renilla luciferase (RLuc) and coelenterazine h (CLZ, lambda(em) approximately 475 nm) and BRET(2) with the substrate coelenterazine 400a (CLZ400A substrate, lambda(em)=395 nm) as the respective donors. For BRET(1) the acceptor is yellow fluorescent protein (YFP) (lambda(em) approximately 535 nm), a mutant of green fluorescent protein (GFP), and for BRET(2) it is GFP(2) (lambda(em) approximately 515 nm). It is not clear from previous studies which of these systems has superior signal-to-background characteristics. Here we directly compared BRET(1) and BRET(2) by placing two different protease-specific cleavage sequences between the donor and acceptor domains. The intact proteins simulate protein-protein association. Proteolytic cleavage of the peptide linker simulates protein dissociation and can be detected as a change in the BRET ratios. Complete cleavage of its target sequence by thrombin changed the BRET(2) ratio by a factor of 28.9+/-0.2 (relative standard deviation [RSD], n=3) and changed the BRET(1) ratio by a factor of 3.05+/-0.07. Complete cleavage of a caspase-3 target sequence resulted in the BRET ratio changes by factors of 15.45+/-0.08 for BRET(2) and 2.00+/-0.04 for BRET(1). The BRET(2) assay for thrombin was 2.9 times more sensitive compared with the BRET(1) version. Calculated detection limits (blank signal+3sigma(b), where sigma(b)=standard deviation [SD] of blank signal) were 53 pM (0.002 U) thrombin with BRET(1) and 15 pM (0.0005 U) thrombin with BRET(2). The results presented here suggest that BRET(2) is a more suitable system than BRET(1) for studying protein-protein interactions and as a potential sensor for monitoring protease activity.

Arrestin binds to different phosphorylated regions of the thyrotropin-releasing hormone receptor with distinct functional consequences

Arrestin binding to agonist-occupied phosphorylated G protein-coupled receptors typically increases the affinity of agonist binding, increases resistance of receptor-bound agonist to removal with high acid/salt buffer, and leads to receptor desensitization and internalization. We tested whether thyrotropin-releasing hormone (TRH) receptors lacking phosphosites in the C-terminal tail could form stable and functional complexes with arrestin. Fibroblasts from mice lacking arrestins 2 and 3 were used to distinguish between arrestin-dependent and -independent effects. Arrestin did not promote internalization or desensitization of a receptor that had key Ser/Thr phosphosites mutated to Ala (4Ala receptor). Nevertheless, arrestin greatly increased acid/salt resistance and the affinity of 4Ala receptor for TRH. Truncation of 4Ala receptor just distal to the key phosphosites (4AlaStop receptor) abolished arrestin-dependent acid/salt resistance but not the effect of arrestin on agonist affinity. Arrestin formed stable complexes with activated wild-type and 4Ala receptors but not with 4AlaStop receptor, as measured by translocation of arrestin-green fluorescent protein to the plasma membrane or chemical cross-linking. An arrestin mutant that does not interact with clathrin and AP2 did not internalize receptor but still promoted high affinity TRH binding, acid/salt resistance, and desensitization. A sterically restricted arrestin mutant did not cause receptor internalization or desensitization but did promote acid/salt resistance and high agonist affinity. The results demonstrate that arrestin binds to proximal or distal phosphosites in the receptor tail. Arrestin binding at either site causes increased agonist affinity and acid/salt resistance, but only the proximal phosphosites evoke the necessary conformational changes in arrestin for receptor desensitization and internalization.