Src promotes delta opioid receptor (DOR) desensitization by interfering with receptor recycling

Can Src protein tyrosine kinase inhibitors be combined with opioid analgesics? Src and opioid-induced tolerance, hyperalgesia and addiction

The clinical application of opioids may be accompanied by a series of adverse consequences, such as opioid tolerance, opioid-induced hyperalgesia, opioid dependence or addiction. In view of this issue, clinicians are faced with the dilemma of treating various types of pain with or without opioids. In this review, we discuss that Src protein tyrosine kinase plays an important role in these adverse consequences, and Src inhibitors can solve these problems well. Therefore, Src inhibitors have the potential to be used in combination with opioids to achieve synergy. How to combine them together to maximize the analgesic effect while avoiding unnecessary trouble provides a topic for follow-up research.

Signaling diversity of mu- and delta- opioid receptor ligands: Re-evaluating the benefits of β-arrestin/G protein signaling bias

Opioid analgesics are elective for treating moderate to severe pain but their use is restricted by severe side effects. Signaling bias has been proposed as a viable means for improving this situation. To exploit this opportunity, continuous efforts are devoted to understand how ligand-specific modulations of receptor functions could mediate the different in vivo effects of opioids. Advances in the field have led to the development of biased agonists based on hypotheses that allocated desired and undesired effects to specific signaling pathways. However, the prevalent hypothesis associating β-arrestin to opioid side effects was recently challenged and multiple of the newly developed biased drugs may not display the superior side effects profile that was sought. Moreover, biased agonism at opioid receptors is now known to be time- and cell-dependent, which adds a new layer of complexity for bias estimation. Here, we first review the signaling mechanisms underlying desired and undesired effects of opioids. We then describe biased agonism at opioid receptors and discuss the different perspectives that support the desired and undesired effects of opioids in view of exploiting biased signaling for therapeutic purposes. Finally, we explore how signaling kinetics and cellular background can influence the magnitude and directionality of bias at those receptors.

Delta opioid receptors recycle to the membrane after sorting to the degradation path

Soon after internalization delta opioid receptors (DOPrs) are committed to the degradation path by G protein-coupled receptor (GPCR)-associated binding protein. Here we provide evidence that this classical post-endocytic itinerary may be rectified by downstream sorting decisions which allow DOPrs to regain to the membrane after having reached late endosomes (LE). The LE sorting mechanism involved ESCRT accessory protein Alix and the TIP47/Rab9 retrieval complex which supported translocation of the receptor to the TGN, from where it subsequently regained the cell membrane. Preventing DOPrs from completing this itinerary precipitated acute analgesic tolerance to the agonist DPDPE, supporting the relevance of this recycling path in maintaining the analgesic response by this receptor. Taken together, these findings reveal a post-endocytic itinerary where GPCRs that have been sorted for degradation can still recycle to the membrane.

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.

Kir3 channels undergo arrestin-dependant internalization following delta opioid receptor activation

Kir3 channels control excitability in the nervous system and the heart. Their surface expression is strictly regulated, but mechanisms responsible for channel removal from the membrane remain incompletely understood. Using transfected cells, we show that Kir3.1/3.2 channels and delta opioid receptors (DORs) associate in a complex which persists during receptor activation, behaving as a scaffold that allows beta-arrestin (βarr) to interact with both signaling partners. This organization favored co-internalization of DORs and Kir3 channels in a βarr-dependent manner via a clathrin/dynamin-mediated endocytic path. Taken together, these findings identify a new way of modulating Kir3 channel availability at the membrane and assign a putatively novel role for βarrs in regulating canonical effectors for G protein-coupled receptors.

Opioid and chemokine receptor crosstalk: a promising target for pain therapy?

Chemokines and opioids are important regulators of immune, inflammatory and neuronal responses in peripheral and central pain pathways. Recent studies have provided insights into the functional interactions between chemokine receptors and opioid receptors, and their role in pain modulation. In this Progress article, we discuss how crosstalk between these two systems might provide a molecular and cellular framework for the development of novel analgesic therapies for the management of acute and/or chronic pain.

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.

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.

Src family kinases involved in CXCL12-induced loss of acute morphine analgesia

Functional interactions between the chemokine receptor CXCR4 and opioid receptors have been reported in the brain, leading to a decreased morphine analgesic activity. However the cellular mechanisms responsible for this loss of opioid analgesia are largely unknown. Here we examined whether Src family-kinases (SFK)-linked mechanisms induced by CXCR4 contributed to the loss of acute morphine analgesia and could represent a new physiological anti-opioid signaling pathway. In this way, we showed by immunohistochemistry and western blot that CXCL12 rapidly activated SFK phosphorylation in vitro in primary cultured lumbar rat dorsal root ganglia (DRG) but also in vivo in the DRG and the spinal cord. We showed that SFK activation occurred in a sub population of sensory neurons, in spinal microglia but also in spinal nerve terminals expressing mu-(MOR) and delta-opioid (DOR) receptor. In addition we described that CXCR4 is detected in MOR- and DOR-immunoreactive neurons in the DRG and spinal cord. In vivo, we demonstrated that an intrathecal administration of CXCL12 (1μg) significantly attenuated the subcutaneous morphine (4mg/kg) analgesia. Conversely, pretreatment with a potent CXCR4 antagonist (5μg) significantly enhanced morphine analgesia. Similar effects were obtained after an intrathecal injection of a specific SFK inhibitor, PP2 (10μg). Furthermore, PP2 abrogated CXCL12-induced decrease in morphine analgesia by suppressing SFK activation in the spinal cord. In conclusion, our data highlight that CXCL12-induced loss of acute morphine analgesia is linked to Src family kinases activation.

Endocytic profiles of δ-opioid receptor ligands determine the duration of rapid but not sustained cAMP responses

Traditional assays that monitor cAMP inhibition by opioid receptor ligands require second-messenger accumulation over periods of 10-20 minutes. Since receptor regulation occurs within a similar time frame, such assays do not discriminate the actual signal from its modulation. Here we used bioluminescence resonance energy transfer to monitor inhibition of cAMP production by δ-opioid receptor (DOR) agonists in real time. cAMP inhibition elicited by different agonists over a period of 15 minutes was biphasic, with response buildup during the first 6 to 7 minutes, followed by a second phase of response decay or of no further increment. The rate at which the cAMP response disappeared was correlated with operational parameters describing ligand efficiency [log(τ/KA)] to promote Gαi activation, as well as with ligand ability to promote internalization during the time course of the assay. Thus, ligands that displayed low signaling efficiency and poor sequestration(SB235863 ([8R-(4bS*,8aα,8aβ,12bβ)]7,10-dimethyl-1-methoxy-11-(2-ethylpropyl)oxycarbonyl 5,6,7,8,12,12b-hexahydro-(9H)-4,8-methanobenzofuro[3,2-e]pyrrolo[2,3-g]isoquinoline hydrochloride), morphine) had minimal or no response decay. On the other hand, the decay rate was pronounced for deltorphin II, [d-Pen(2), d-pen(5)]-enkephalin, met-enkephalin, and SNC-80 ((+)-4-[(αR)-α-((2S,5R)-4-allyl-2,5-dimethyl-1-piperazinyl)-3-methoxybenzyl]-N,N-diethylbenzamide), which displayed high signaling efficiency and internalization. Moreover, inhibition of internalization by dynasore reduced or abolished response decay by internalizing ligands. Unlike acute responses, endocytic profiles were not predictive of whether an agonist would induce prolonged cAMP inhibition over sustained (30-120 minutes) DOR stimulation. Taken together, the data indicate that ligand ability to evoke G-protein activation or promote endocytosis was predictive of response duration over short, but not over sustained periods of cAMP inhibition.

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.

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.

Fast modulation of μ-opioid receptor (MOR) recycling is mediated by receptor agonists

The μ-opioid receptor (MOR) is a member of the G protein-coupled receptor family and the main target of endogenous opioid neuropeptides and morphine. Upon activation by ligands, MORs are rapidly internalized via clathrin-coated pits in heterologous cells and dissociated striatal neurons. After initial endocytosis, resensitized receptors recycle back to the cell surface by vesicular delivery for subsequent cycles of activation. MOR trafficking has been linked to opioid tolerance after acute exposure to agonist, but it is also involved in the resensitization process. Several studies describe the regulation and mechanism of MOR endocytosis, but little is known about the recycling of resensitized receptors to the cell surface. To study this process, we induced internalization of MOR with [D-Ala(2), N-Me-Phe(4), Gly(5)-ol]-enkephalin (DAMGO) and morphine and imaged in real time single vesicles recycling receptors to the cell surface. We determined single vesicle recycling kinetics and the number of receptors contained in them. Then we demonstrated that rapid vesicular delivery of recycling MORs to the cell surface was mediated by the actin-microtubule cytoskeleton. Recycling was also dependent on Rab4, Rab11, and the Ca(2+)-sensitive motor protein myosin Vb. Finally, we showed that recycling is acutely modulated by the presence of agonists and the levels of cAMP. Our work identifies a novel trafficking mechanism that increases the number of cell surface MORs during acute agonist exposure, effectively reducing the development of opioid tolerance.

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.

Cysteine 27 variant of the delta-opioid receptor affects amyloid precursor protein processing through altered endocytic trafficking

Agonist-induced activation of the δ-opioid receptor (δOR) was recently shown to augment β- and γ-secretase activities, which increased the production of β-amyloid peptide (Aβ), known to accumulate in the brain tissues of Alzheimer's disease (AD) patients. Previously, the δOR variant with a phenylalanine at position 27 (δOR-Phe27) exhibited more efficient receptor maturation and higher stability at the cell surface than did the less common cysteine (δOR-Cys27) variant. For this study, we expressed these variants in human SH-SY5Y and HEK293 cells expressing exogenous or endogenous amyloid precursor protein (APP) and assessed the effects on APP processing. Expression of δOR-Cys27, but not δOR-Phe27, resulted in a robust accumulation of the APP C83 C-terminal fragment and the APP intracellular domain, while the total soluble APP and, particularly, the β-amyloid 40 levels were decreased. These changes upon δOR-Cys27 expression coincided with decreased localization of APP C-terminal fragments in late endosomes and lysosomes. Importantly, a long-term treatment with a subset of δOR-specific ligands or a c-Src tyrosine kinase inhibitor suppressed the δOR-Cys27-induced APP phenotype. These data suggest that an increased constitutive internalization and/or concurrent signaling of the δOR-Cys27 variant affects APP processing through altered endocytic trafficking of APP.

Heteromerization of the μ- and δ-opioid receptors produces ligand-biased antagonism and alters μ-receptor trafficking

Heteromerization of opioid receptors has been shown to alter opioid receptor pharmacology. However, how receptor heteromerization affects the processes of endocytosis and postendocytic sorting has not been closely examined. This question is of particular relevance for heteromers of the μ-opioid receptor (MOR) and δ-opioid receptor (DOR), because the MOR is recycled primarily after endocytosis and the DOR is degraded in the lysosome. Here, we examined the endocytic and postendocytic fate of MORs, DORs, and DOR/MOR heteromers in human embryonic kidney 293 cells stably expressing each receptor alone or coexpressing both receptors. We found that the clinically relevant MOR agonist methadone promotes endocytosis of MOR but also the DOR/MOR heteromer. Furthermore, we show that DOR/MOR heteromers that are endocytosed in response to methadone are targeted for degradation, whereas MORs in the same cell are significantly more stable. It is noteworthy that we found that the DOR-selective antagonist naltriben mesylate could block both methadone- and [D-Ala2,NMe-Phe4,Gly-ol5]-enkephalin-induced endocytosis of the DOR/MOR heteromers but did not block signaling from this heteromer. Together, our results suggest that the MOR adopts novel trafficking properties in the context of the DOR/MOR heteromer. In addition, they suggest that the heteromer shows "biased antagonism," whereby DOR antagonist can inhibit trafficking but not signaling of the DOR/MOR heteromer.

[Functional selectivity of opioid receptors ligands]

Opiates are the most effective analgesics available for the treatment of severe pain. However, their clinical use is restricted by unwanted side effects such as tolerance, physical dependence and respiratory depression. The strategy to develop new opiates with reduced side effects has mainly focused on the study and production of ligands that specifically bind to different opiate receptors subtypes. However, this strategy has not allowed the production of novel therapeutic ligands with a better side effects profile. Thus, other research strategies need to be explored. One which is receiving increasing attention is the possibility of exploiting ligand ability to stabilize different receptor conformations with distinct signalling profiles. This newly described property, termed functional selectivity, provides a potential means of directing the stimulus generated by an activated receptor towards a specific cellular response. Here we summarize evidence supporting the existence of ligand-specific active conformations for two opioid receptors subtypes (delta and mu), and analyze how functional selectivity may contribute in the production of longer lasting, better tolerated opiate analgesics. double dagger.

Endogenous opiates and behavior: 2009

This paper is the 32nd consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2009 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 (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

Multi-component signaling complexes of the delta-opioid receptor with STAT5B and G proteins

Besides mediating opioid responses in the nervous system and the peripheral tissues, opioid receptors are implicated in signaling mechanisms shared by cytokine receptors. Recent observations have shown that the Signal Transducer and Activator of Transcription 5A (STAT5A) interacts with the mu-opioid receptor (mu-OR) and is phosphorylated upon mu-OR stimulation (Mazarakou and Georgoussi, 2005). In the present study we demonstrate that another member of the STAT family, STAT5B, associates constitutively with the C-terminal tail of the delta-opioid receptor (delta-CT). [D-Ser(2), Leu(5), Thr(6)]-enkephalin-exposure of HEK293 cells, expressing stably the delta-opioid receptor (delta-OR), leads to receptor-dependent STAT5B tyrosine phosphorylation and transcriptional activation. This phosphorylation occurs in a G protein-dependent manner and is carried out by a c-Src kinase. Co-immunoprecipitation studies indicate that STAT5B forms pairs with selective Galpha and Gbetagamma subunits of G proteins and activated c-Src kinase in HEK293 cells. These interactions are formed either constitutively, or upon receptor stimulation. We also demonstrate that the delta-CT serves as a platform for the formation of a multi-component signaling complex (signalosome), consisting of STAT5B, c-Src and selective G protein members. We can thus conclude that STAT5B signaling can be modulated by its coupling with a specific subset of G protein subunits, revealing a novel signaling mechanism for the transcriptional regulation of STAT5B-dependent genes.

Further studies on lead compounds containing the opioid pharmacophore Dmt-Tic

Some reference opioids containing the Dmt-Tic pharmacophore, especially the delta agonists H-Dmt-Tic-Gly-NH-Ph (1) and H-Dmt-Tic-NH-(S)CH(CH2-COOH)-Bid (4) (UFP-512) were evaluated for the influence of the substitution of Gly with aspartic acid, its chirality, and the importance of the -NH-Ph and N(1)H-Bid hydrogens in the inductions of delta agonism. The results provide the following conclusions: (i) Asp increases delta selectivity by lowering the mu affinity; (ii) -NH-Ph and N(1)H-Bid nitrogens methylation transforms the delta agonists into delta antagonists; (iii) the substitution of Gly with L-Asp/D-Asp in the delta agonist H-Dmt-Tic-Gly-NH-Ph gave delta antagonists; the same substitution in the delta agonist H-Dmt-Tic-NH-CH2-Bid yielded more selective agonists, H-Dmt-Tic-NH-(S)CH(CH2-COOH)-Bid and H-Dmt-Tic-NH-(R)CH(CH2-COOH)-Bid; (iv) L-Asp seems important only in functional bioactivity, not in receptor affinity; (v) H-Dmt-Tic-NH-(S)CH(CH2-COOH)-Bid(N(1)-Me) (10) evidenced analgesia similar to 4, which was reversed by naltrindole only in the tail flick. 4 and 10 had opposite behaviours in mice; 4 caused agitation, 10 gave sedation and convulsions.