Phosphorylation is not required for dynamin-dependent endocytosis of a truncated mutant opioid receptor

A Genetically Encoded Biosensor Reveals Location Bias of Opioid Drug Action

Opioid receptors (ORs) precisely modulate behavior when activated by native peptide ligands but distort behaviors to produce pathology when activated by non-peptide drugs. A fundamental question is how drugs differ from peptides in their actions on target neurons. Here, we show that drugs differ in the subcellular location at which they activate ORs. We develop a genetically encoded biosensor that directly detects ligand-induced activation of ORs and uncover a real-time map of the spatiotemporal organization of OR activation in living neurons. Peptide agonists produce a characteristic activation pattern initiated in the plasma membrane and propagating to endosomes after receptor internalization. Drugs produce a different activation pattern by additionally driving OR activation in the somatic Golgi apparatus and Golgi elements extending throughout the dendritic arbor. These results establish an approach to probe the cellular basis of neuromodulation and reveal that drugs distort the spatiotemporal landscape of neuronal OR activation.

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.

Role of Phosphorylation in the Control of Clathrin-Mediated Internalization of GPCR

The process by which G protein-coupled receptors (GPCRs) are internalized through the clathrin-coated vesicles involves interactions of multifunctional adaptor proteins. These interactions are tightly controlled by phosphorylation and dephosphorylation mechanisms resulting in the regulation of receptor endocytosis. However, the identities of the kinases involved in this process remained largely unknown until recently. This paper discusses advances in our knowledge of the important role played by protein phosphorylation in the regulation of the endocytic machinery and how phosphorylation controls the coated vesicle cycle.

Phosphorylation of protease-activated receptor-2 differentially regulates desensitization and internalization

Protease-activated receptor 2 (PAR2) is a G protein-coupled receptor irreversibly activated by extracellular proteases. Activated PAR2 couples to multiple heterotrimeric G-protein subtypes including G alpha(q), G alpha(i), and G alpha(12/13). Most activated G protein-coupled receptors are rapidly desensitized and internalized following phosphorylation and beta-arrestin binding. However, the role of phosphorylation in regulation of PAR2 signaling and trafficking is not known. To investigate the function of phosphorylation, we generated a PAR2 mutant in which all serines and threonines in the C-tail were converted to alanines and designated it PAR2 0P. In mammalian cells, the addition of agonist induced a rapid and robust increase in phosphorylation of wild-type PAR2 but not the 0P mutant, suggesting that the major sites of phosphorylation occur within the C-tail domain. Moreover, desensitization of PAR2 0P signaling was markedly impaired compared with the wild-type receptor. Wild-type phosphorylated PAR2 internalized through a canonical dynamin, clathrin- and beta-arrestin-dependent pathway. Strikingly, PAR2 0P mutant internalization proceeded through a dynamin-dependent but clathrin- and beta-arrestin-independent pathway in both a constitutive and agonist-dependent manner. Collectively, our studies show that PAR2 phosphorylation is essential for beta-arrestin binding and uncoupling from heterotrimeric G-protein signaling and that the presence of serine and threonine residues in the PAR2 C-tail hinder constitutive internalization through a non-canonical pathway. Thus, our studies reveal a novel function for phosphorylation that differentially regulates PAR2 desensitization and endocytic trafficking.

G protein independent phosphorylation and internalization of the delta-opioid receptor

Agonist activation of the delta-opioid receptor leads to internalization via G betagamma recruitment of G protein coupled receptor kinase-2, which phosphorylates the receptor at several sites, including Ser363, allowing beta-arrestin binding and localization to clathrin coated pits. Using human embryonic kidney cells expressing a delta-opioid receptor we tested the hypothesis that prevention of receptor coupling to G protein by treatment with pertussis toxin (PTX) will block these processes. PTX treatment did not reduce phosphorylation of delta-opioid receptor Ser363 in response to the agonist [D-Pen2, D-Pen5]enkephalin, or recruitment of beta-arrestin 2-green fluorescent protein to the membrane and only slowed, but did not prevent, [D-Pen2, D-Pen5]enkephalin-induced internalization. Similarly, PTX treatment only partially prevented the ability of the delta-opioid peptide agonists deltorphin II and [Met5]enkephalin and the non-peptide agonist BW373U86 to induce receptor internalization. No internalization was seen with morphine, oxymorphindole or the putative delta(1)-opioid agonist TAN-67 in the presence or absence of PTX, even though TAN-67 showed a strong activation of G protein, as measured by guanosine-5'-O-(3-[(35)S]thio)triphosphate binding. The ability of an agonist to stimulate phosphorylation at Ser363 was predictive of its capacity to induce internalization. The results suggest a role for G protein in delta-opioid receptor internalization, but show that alternative G protein independent pathways exist.

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

Abstract An important limitation in the clinical use of opiates is progressive loss of analgesic efficacy over time. Development of analgesic tolerance is tightly linked to receptor desensitization. In the case of delta opioid receptors (DOR), desensitization is especially swift because receptors are rapidly internalized and are poorly recycled to the membrane. In the present study, we investigated whether Src activity contributed to this sorting pattern and to functional desensitization of DORs. A first series of experiments demonstrated that agonist binding activates Src and destabilizes a constitutive complex formed by the spontaneous association of DORs with the kinase. Src contribution to DOR desensitization was then established by showing that pre-treatment with Src inhibitor PP2 (20 microM; 1 hr) or transfection of a dominant negative Src mutant preserved DOR signalling following sustained exposure to an agonist. This protection was afforded without interfering with endocytosis, but suboptimal internalization interfered with PP2 ability to preserve DOR signalling, suggesting a post-endocytic site of action for the kinase. This assumption was confirmed by demonstrating that Src inhibition by PP2 or its silencing by siRNA increased membrane recovery of internalized DORs and was further corroborated by showing that inhibition of recycling by monensin or dominant negative Rab11 (Rab11S25N) abolished the ability of Src blockers to prevent desensitization. Finally, Src inhibitors accelerated recovery of DOR-Galphal3 coupling after desensitization. Taken together, these results indicate that Src dynamically regulates DOR recycling and by doing so contributes to desensitization of these receptors.

Desensitization and internalization of the human motilin receptor is independent of the C-terminal tail

The motilin receptor (MTLR) is an important therapeutic target for the treatment of hypomotility disorders but desensitization may limit its clinical utility. The aim of this study was to investigate the role of the C-terminal tail of the MTLR in the desensitization, phosphorylation and internalization process. Three MTLR mutants, C-terminally truncated from amino acid 412 till 384 (MTLRDelta385), 374 (MTLRDelta375) or 368 (MTLRDelta369), were constructed and C-terminally tagged with an EGFP and stably expressed in CHO cells co-expressing the Ca(2+) indicator apoaequorin. Activity and desensitization were studied by measuring changes in motilin-induced luminescent Ca(2+) rises. Receptor phosphorylation was investigated by immunoprecipitation and MTLR-EGFP internalization was visualized by fluorescence microscopy. Truncation only reduced MTLR affinity and the efficacy to induce Ca(2+) luminescent responses of the MTLRDelta375-EGFP mutant. Furthermore, the region between amino acid 375 and 368 seems to be important for proper cell surface expression of the MTLR since receptors of the MTLRDelta369-EGFP mutant but not of the other mutants were found intracellularly in vesicles. Truncation of the receptor till amino acid 384 or 374 did neither affect desensitization nor internalization. In contrast phosphorylation of the MTLRDelta385-EGFP mutant was reduced by 80% but was not affected in the MTLRDelta375-EGFP mutant. In conclusion, MTLR desensitization and internalization is not dependent on the presence of the C-terminal tail. Truncation favors internalization via either phosphorylation-independent pathways or via phosphorylation of alternative sites in the receptor.

Regulation of CB1 cannabinoid receptor internalization by a promiscuous phosphorylation-dependent mechanism

Agonists stimulate cannabinoid 1 receptor (CB(1)R) internalization. Previous work suggests that the extreme carboxy-terminus of the receptor regulates this internalization - likely through the phosphorylation of serines and threonines clustered within this region. While truncation of the carboxy-terminus (V460Z CB(1)) and consequent removal of these putative phosphorylation sites prevents endocytosis in AtT20 cells, the residues necessary for CB(1)R internalization remain elusive. To determine the structural requirements for internalization, we evaluated endocytosis of carboxy-terminal mutant CB(1)Rs stably expressed in HEK293 cells. In contrast to AtT20 cells, V460Z CB(1)R expressed in HEK293 cells internalized to the same extent and with similar kinetics as the wild-type receptor. However, mutation of serine and/or threonine residues within the extreme carboxy-terminal attenuated internalization when these receptors were expressed in HEK293 cells. These results establish that the extreme carboxy-terminal phosphorylation sites are not required for internalization of truncated receptors, but are required for internalization of full-length receptors in HEK293 cells. Analysis of beta-arrestin-2 recruitment to mutant CB(1)R suggests that putative carboxy-terminal phosphorylation sites mediate beta-arrestin-2 translocation. This study indicates that the local cellular environment affects the structural determinants of CB(1)R internalization. Additionally, phosphorylation likely regulates the internalization of (full-length) CB(1)Rs.

Phosphorylation state of mu-opioid receptor determines the alternative recycling of receptor via Rab4 or Rab11 pathway

Agonist-induced phosphorylation, internalization, and intracellular trafficking of G protein-coupled receptors are critical in regulating both cellular responsiveness and signal transduction. The current study investigated the role of receptor phosphorylation state in regulation of agonist-induced internalization and intracellular trafficking of mu-opioid receptor (MOR). Our results showed that after agonist stimulation, the recycle of a mutant MOR that lacks the C-terminal residues after Asn(362) (MOR362T) was greatly decreased, whereas a C-terminal phosphorylation sites-mutated MOR (MOR3A), which is deficient in agonist-induced phosphorylation recycled back to the membrane at a level comparable to that of the wild-type receptor, however, interestingly at a slower rate. Inhibition of functions of either Rab4 or Rab11 by dominant-negative mutants and small interfering RNA both significantly impaired the recycling of the wild-type MOR, whereas the recycling of the phosphorylation-deficient mutant was only inhibited by the dominant-negative mutant and small interfering RNA of Rab11, suggesting that the recycling of nonphosphorylated MOR is exclusively via Rab11-mediated pathway. Furthermore, phosphorylated MOR was observed accumulated in Rab5- and Rab4-, but not Rab11-positive vesicles. Our data indicate that both phosphorylated and nonphosphorylated MOR internalize via Rab5-dependent pathway after agonist stimulation, and the phosphorylated and nonphosphorylated MORs recycle through distinct vesicular trafficking pathways mediated by Rab4 and Rab11, respectively, which may ultimately lead to differential cellular responsiveness or downstream signaling.

Agonist-specific down regulation of mu-opioid receptors: Different cellular pathways are activated by different opioid agonists

Opioid agonists are known to induce down regulation of opioid receptors through the classical pathway that involves phosphorylation, clathrin-dependent endocytosis and lysosomal/endosomal degradation of the internalized receptors. As expected, exposure of mu-opioid receptor (MOR)-transfected HEK-293 cells to either DAMGO (a specific mu-opioid agonist) or etorphine (a wide spectrum opioid agonist) resulted in down regulation of the receptors that was blocked by the kinase inhibitor staurosporine, by hypertonic sucrose and by the lysosomal and proteasomal inhibitors chloroquine and lactacystin. High concentration of etorphine, but not of DAMGO, induced an additional process of down regulation that was resistant to staurosporine, to hypertonic sucrose and to chloroquine-lactacystin. Etorphine, but not DAMGO, also induced down regulation of mu-opioid receptors in isolated membranes of HEK cells. This membrane-delimited down regulation was blocked by selective inhibitors of protease enzymes, suggesting the involvement of membranous serine- and amino-peptidases. This membranous down regulation of opioid receptors was dependent on the concentration of etorphine and was blocked by the opioid antagonist naloxone. Etorphine induced similar down regulation in membranes of HEK-293 cells transfected with delta-opioid receptors (DOR) as well in membranes of cells that endogenously express opioid receptors. This agonist-specific membrane-delimited regulatory process appears to be physiologically relevant and should be taken into account when studying long term effects of opioid drugs.

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

In the current study, we investigated the role of receptor phosphorylation and beta-arrestins in delta-opioid receptor (DOR) signaling and trafficking by using a DOR mutant in which all Ser/Thr residues in the C terminus were mutated to Ala (DTS). We demonstrated that the DOR agonist D-[Pen(2),Pen(5)]enkephalin could induce receptor internalization and adenylyl cyclase (AC) desensitization of DTS, but with comparatively slower kinetics than those observed with wild type DOR. Blockade of the internalization of DTS by the dominant-negative mutant dynamin, dynamin K44E, did not affect AC desensitization. However, depletion of beta-arrestins almost totally blocked both internalization and AC desensitization of DTS. A BRET assay suggested that DOR phosphorylation promotes receptor selectivity for beta-arrestin 2 over beta-arrestin 1. Furthermore, in mouse embryonic fibroblast (MEF) cells lacking either beta-arrestin 1 (beta arr1(-/-)) or beta-arrestin 2 (beta arr2(-/-)), agonist-induced DTS desensitization and internalization were similar to that observed in wild type MEFs. In contrast, although DOR internalization decreased in both beta arr1(-/-) MEFs and beta arr2(-/-) MEFs, DPDPE-induced DOR desensitization was significantly reduced in beta arr2(-/-) MEFs, but not in beta arr1(-/-) MEFs. Additionally, the BRET assay suggested that depletion of phosphorylation did not influence the stability of the receptor-beta-arrestin complex. Consistent with this observation, DTS did not recycle after internalization, which is like wild type DOR. Taken together, these results indicate that receptor phosphorylation confers DOR selectivity for beta-arrestin 2 without affecting the stability of the receptor-beta-arrestin complex and the fate of the internalized receptor.

Membrane glycoprotein M6a interacts with the micro-opioid receptor and facilitates receptor endocytosis and recycling

Using a yeast two-hybrid screen, the neuronal membrane glycoprotein M6a, a member of the proteolipid protein family, was identified to be associated with the mu-opioid receptor (MOPr). Bioluminescence resonance energy transfer and co-immunoprecipitation experiments confirmed that M6a interacts agonist-independently with MOPr in human embryonic kidney 293 cells co-expressing MOPr and M6a. Co-expression of MOPr with M6a, but not with M6b or DM20, exists in many brain regions, further supporting a specific interaction between MOPr and M6a. After opioid treatment M6a co-internalizes and then co-recycles with MOPr to cell surface in transfected human embryonic kidney 293 cells. Moreover, the interaction of M6a and MOPr augments constitutive and agonist-dependent internalization as well as the recycling rate of mu-opioid receptors. On the other hand, overexpression of a M6a-negative mutant prevents mu-opioid receptor endocytosis, demonstrating an essential role of M6a in receptor internalization. In addition, we demonstrated the interaction of M6a with a number of other G protein-coupled receptors (GPCRs) such as the delta-opioid receptor, cannabinoid receptor CB1, and somatostatin receptor sst2A, suggesting that M6a might play a general role in the regulation of certain GPCRs. Taken together, these data provide evidence that M6a may act as a scaffolding molecule in the regulation of GPCR endocytosis and intracellular trafficking.

Ligand-specific receptor states: Implications for opiate receptor signalling and regulation

Opiate drugs produce their effects by acting upon G protein coupled receptors (GPCRs) and although they are among the most effective analgesics available, their clinical use is restricted by unwanted side effects such as tolerance, physical dependence, respiratory depression, nausea and constipation. As a class, opiates share a common profile of unwanted effects but there are also significant differences in ligand liability for producing these actions. A growing number of studies show that GPCRs may exist in multiple active states that differ in their signalling and regulatory properties and which may distinctively bind different agonists. In this review we summarize evidence supporting the existence of multiple active conformations for MORs and DORs, analyze information favouring the existence of ligand-specific receptor states and assess how ligand-selective efficacy may contribute to the production of longer lasting, better tolerated opiate analgesics.

Tracking the opioid receptors on the way of desensitization

Opioid receptors belong to the super family of G-protein coupled receptors (GPCRs) and are the targets of numerous opioid analgesic drugs. Prolonged use of these drugs results in a reduction of their effectiveness in pain relief also called tolerance, a phenomenon well known by physicians. Opioid receptor desensitization is thought to play a major role in tolerance and a lot of work has been dedicated to elucidate the molecular basis of desensitization. As described for most of GPCRs, opioid receptor desensitization involves their phosphorylation by kinases and their uncoupling from G-proteins realized by arrestins. More recently, opioid receptor trafficking was shown to contribute to desensitization. In this review, our knowledge on the molecular mechanisms of desensitization and recent progress on the role of opioid receptor internalization, recycling or degradation in desensitization will be reported. A better understanding of these regulatory mechanisms would be helpful to develop new analgesic drugs or new strategies for pain treatment by limiting opioid receptor desensitization and tolerance.

High-purity selection and maintenance of gene expression in human neuroblastoma cells stably over-expressing GFP fusion protein. Application for opioid receptors desensitization studies

Chronic use of opiates such as morphine is associated with drug tolerance, which is correlated with the desensitization of opioid receptors. This latter process involves phosphorylation of opioid receptors by G protein-coupled receptors kinases (GRKs) and subsequent uncoupling by beta-arrestins. To explore these molecular mechanisms, neuronal cell lines, endogenously expressing the opioid receptors, provide an ideal cellular model. Unfortunately, there are two major drawbacks: (1) these cells are refractory to cDNA introduction, resulting in low transfection efficiency; (2) continuous culturing of transfected cells invariably leads to phenotypic drift of the cultures even after an antibiotic selection. So, these cells were dropped in favor of heterologous expression systems, which are easier to transfect but whose relevance as adequate cellular model for studying opioid receptor regulation should be questioned, as recently demonstrated by [Haberstock-Debic, H., Kim, K.A.,Yu, Y.J., von Zastrow, M., 2005. Morphine promotes rapid, arrestin-dependent endocytosis of mu-opioid receptors in striatal neurons. J. Neurosci. 25, 7847-7857]. In this work, we describe a method, based on fluorescence-activated cell sorting (FACS), to select and maintain a high proportion of transfected SK-N-BE cells (a neuronal cell line endogenously expressing human Delta-Opioid Receptor (hDOR)), expressing the beta-arrestin1 fused to green fluorescent protein (GFP). While in functional experiments, we were not able to observe a major effect in non-sorted SK-N-BE cells expressing beta-arrestin1-GFP, the enrichment by 18-fold with FACS resulted in a robust increase of beta-arrestin1-GFP expression associated with strong hDOR desensitization. Moreover, this method also allows to counteract the phenotypic drift and to maintain a high-purity selection of SK-N-BE cells expressing beta-arrestin1-GFP. Thus, this approach provides a valuable tool for exploring opioid receptors desensitization in neuronal cells.

Beta-arrestin1 and beta-arrestin2 are differentially required for phosphorylation-dependent and -independent internalization of delta-opioid receptors

Beta-arrestins are key negative regulators and scaffolds of G protein-coupled receptor (GPCR) signalling. Beta-arrestin1 and beta-arrestin2 preferentially bind to the phosphorylated GPCRs in response to agonist stimulation, resulting in receptor internalization and desensitization. The critical roles of GPCR kinases (GRKs)-catalyzed receptor phosphorylation and interaction of beta-arrestins with the phosphorylated receptor in receptor internalization are well established. However, emerging evidence suggests that an agonist-stimulated internalization mechanism that is independent of receptor phosphorylation may also be employed in some cases, although the molecular mechanism for the phosphorylation-independent GPCR internalization is not clear. The current study investigated the role of receptor phosphorylation and the involvement of different beta-arrestin subtypes in agonist-induced delta-opioid receptor (DOR) internalization in HEK293 cells. Results from flow cytometry, fluorescence microscopy, and surface biotin labelling experiments showed that elimination of agonist-induced DOR phosphorylation by mutation GRK binding or phosphorylation sites only partially blocked agonist-induced receptor internalization, indicating the presence of an agonist-induced, GRK-independent mechanism for DOR internalization. Fluorescence and co-immunoprecipitation studies indicated that both the wild-type DOR and the phosphorylation-deficient mutant receptor could bind and recruit beta-arrestin1 and beta-arrestin2 to the plasma membrane in an agonist-stimulated manner. Furthermore, internalization of both the wild-type and phosphorylation-deficient receptors was increased by overexpression of either type of beta-arrestins and blocked by dominant-negative mutants of beta-arrestin-mediated internalization, demonstrating that both phosphorylation-dependent and -independent internalization require beta-arrestin. Moreover, double-stranded RNA-mediated interference experiments showed that either beta-arrestin1 or beta-arrestin2 subtype-specific RNAi only partially inhibited agonist-induced internalization of the wild-type DOR. However, agonist-induced internalization of the phosphorylation-deficient DOR was not affected by beta-arrestin1-specific RNAi but was blocked by RNAi against beta-arrestin2 subtype. These data indicate that endogenous beta-arrestin1 functions exclusively in the phosphorylation-dependent receptor internalization, whereas endogenous beta-arrestin2, but not beta-arrestin1, is required for the phosphorylation-independent receptor internalization. These results thus provide the first evidence of different requirement for beta-arrestin isoforms in the agonist induced phosphorylation-dependent and -independent GPCR internalization.

Agonist-specific regulation of the delta-opioid receptor

Delta opioid receptor (DOR) agonists are attractive potential analgesics, since these compounds exhibit strong antinociceptive activity with relatively few side effects. In the past decade, several novel classes of delta-opioid agonists have been synthesized. Recent experimental data indicate that structurally distinct opioid agonists interact differently with the delta-opioid receptor. Consequently, individual agonist-bound DOR conformations may interact differently with intracellular proteins. In the present paper, after a brief review of the cellular processes that contribute to homologous desensitization of the DOR signaling, we shall focus on experimental data demonstrating that chemically different agonists differ in their ability to phosphorylate, internalize, and/or down-regulate the DOR. Homologous regulation of the opioid receptor signaling is thought to play an important role in the development of opioid tolerance. Therefore, agonist-specific differences in DOR regulation suggest that by further chemical modification, delta-selective opioid analgesics can be designed that exhibit a reduced propensity for analgesic tolerance.

Insights into the receptor transcription and signaling: implications in opioid tolerance and dependence

Drug addiction has great social and economical implications. In order to resolve this problem, the molecular and cellular basis for drug addiction must be elucidated. For the past three decades, our research has focused on elucidating the molecular mechanisms behind morphine tolerance and dependence. Although there are many working hypotheses, it is our premise that cellular modulation of the receptor signaling, either via transcriptional or post-translational control of the receptor, is the basis for morphine tolerance and dependence. Thus, in the current review, we will summarize our recent work on the transcriptional and post-translational control of the opioid receptor, with special emphasis on the mu-opioid receptor, which is demonstrated to mediate the in vivo functions of morphine.

Opioid receptors

Opioid receptors belong to the large superfamily of seven transmembrane-spanning (7TM) G protein-coupled receptors (GPCRs). As a class, GPCRs are of fundamental physiological importance mediating the actions of the majority of known neurotransmitters and hormones. Opioid receptors are particularly intriguing members of this receptor family. They are activated both by endogenously produced opioid peptides and by exogenously administered opiate compounds, some of which are not only among the most effective analgesics known but also highly addictive drugs of abuse. A fundamental question in addiction biology is why exogenous opioid drugs, such as morphine and heroin, have a high liability for inducing tolerance, dependence, and addiction. This review focuses on many aspects of opioid receptors with the aim of gaining a greater insight into mechanisms of opioid tolerance and dependence.

Identification of a novel endocytic recycling signal in the D1 dopamine receptor

A critical event determining the functional consequences of G protein-coupled receptor (GPCR) endocytosis is the molecular sorting of internalized receptors between divergent recycling and degradative membrane pathways. The D1 dopamine receptor recycles rapidly and efficiently to the plasma membrane after agonist-induced endocytosis and is remarkably resistant to proteolytic down-regulation. Whereas the mechanism mediating agonist-induced endocytosis of D1 receptors has been investigated in some detail, little is known about how receptors are sorted after endocytosis. We have identified a sequence present in the carboxyl-terminal cytoplasmic domain of the human D1 dopamine receptor that is specifically required for the efficient recycling of endocytosed receptors back to the plasma membrane. This sequence is distinct from previously identified membrane trafficking signals and is located in a proximal portion of the carboxyl-terminal cytoplasmic domain, in contrast to previously identified GPCR recycling signals present at the distal tip. Nevertheless, fusion of this sequence to the carboxyl terminus of a chimeric mutant delta opioid neuropeptide receptor is sufficient to re-route internalized receptors from lysosomal to recycling membrane pathways, defining this sequence as a bona fide endocytic recycling signal that can function in both proximal and distal locations. These results identify a novel sorting signal controlling the endocytic trafficking itinerary of a physiologically important dopamine receptor, provide the first example of such a sorting signal functioning in a proximal portion of the carboxyl-terminal cytoplasmic domain, and suggest the existence of a diverse array of sorting signals in the GPCR superfamily that mediate subtype-specific regulation of receptors via endocytic membrane trafficking.

The molecular mechanisms of cellular tolerance to delta-opioid agonists. A minireview

Chronic treatment with deltaopioid agonists, similar to other agonist drugs, causes tolerance. Tolerance is a complex adaptation process that consists of multiple, cellular and neural-system adaptations. Cellular tolerance to delta-opioid agonists involves feedback-regulation of the function, concentration, and localization of the delta-opioid receptors (receptor desensitization) as well as of intracellular effectors (functional desensitization). We are using a recombinant Chinese hamster ovary cell line expressing the human delta-opioid receptors (hDOR/CHO) to investigate the molecular mechanisms of cellular tolerance. We found that the structurally distinct delta-opioid agonists mediate receptor down-regulation by different mechanisms. Thus, truncation of the last 35 C-terminal amino acids of the hDOR completely abolished DPDPE, but not SNC 80-mediated receptor down-regulation. In addition, down-regulation of the wild type-, and the truncated hDORs exhibited different inhibitor sensitivity-profile. Chronic delta-opioid agonist treatment also causes functional desensitization of forskolin-stimulated cAMP formation and cAMP overshoot in the hDOR/CHO cells. We have demonstrated that chronic SNC 80 treatment also causes concurrent phosphorylation of the adenylyl cyclase (AC) VI isoenzyme hDOR/CHO cells. Both AC superactivation and AC VI phosphorylation were SNC 80 dose-dependent, naltrindole-sensitive, and exhibited similar time course-, and protein kinase inhibitor-sensitivity profile. We hypothesize that phosphorylation of AC VI plays an important role in delta-opioid agonist-mediated AC superactivation in hDOR/CHO cells.

The role of phosphorylation in D1 dopamine receptor desensitization: evidence for a novel mechanism of arrestin association

Homologous desensitization of D(1) dopamine receptors is thought to occur through their phosphorylation leading to arrestin association which interdicts G protein coupling. In order to identify the relevant domains of receptor phosphorylation, and to determine how this leads to arrestin association, we created a series of mutated D(1) receptor constructs. In one mutant, all of the serine/threonine residues within the 3rd cytoplasmic domain were altered (3rdTOT). A second construct was created in which only three of these serines (serines 256, 258, and 259) were mutated (3rd234). We also created four truncation mutants of the carboxyl terminus (T347, T369, T394, and T404). All of these constructs were comparable with the wild-type receptor with respect to expression and adenylyl cyclase activation. In contrast, both of the 3rd loop mutants exhibited attenuated agonist-induced receptor phosphorylation that was correlated with an impaired desensitization response. Sequential truncation of the carboxyl terminus of the receptor resulted in a sequential loss of agonist-induced phosphorylation. No phosphorylation was observed with the most severely truncated T347 mutant. Surprisingly, all of the truncated receptors exhibited normal desensitization. The ability of the receptor constructs to promote arrestin association was evaluated using arrestin-green fluorescent protein translocation assays and confocal fluorescence microscopy. The 3rd234 mutant receptor was impaired in its ability to induce arrrestin translocation, whereas the T347 mutant was comparable with wild type. Our data suggest a model in which arrestin directly associates with the activated 3rd cytoplasmic domain in an agonist-dependent fashion; however, under basal conditions, this is sterically prevented by the carboxyl terminus of the receptor. Receptor activation promotes the sequential phosphorylation of residues, first within the carboxyl terminus and then the 3rd cytoplasmic loop, thereby dissociating these domains and allowing arrestin to bind to the activated 3rd loop. Thus, the role of receptor phosphorylation is to allow access of arrestin to its receptor binding domain rather than to create an arrestin binding site per se.

Modulation of Fas receptor proteins and dynamin during opiate addiction and induction of opiate withdrawal in rat brain

The Fas receptor is involved in the regulation of apoptosis but also can function as a non-apoptotic signal transducer. This study was mainly designed to quantitate Fas proteins in rat brain during heroin addiction and opiate withdrawal. In rat, mouse and human brains, and in SH-SY5Y cells, similar forms of Fas were immunodetected with different antibodies (i.e., 35 kDa native Fas and 48- and 51-kDa glycosylated Fas). Acute (2 h) treatments with the micro-opioid receptor agonists heroin (10 mg/kg) and morphine (30 mg/kg) increased the immunodensity of native Fas (124% and 36%) but not that of glycosylated Fas in the cerebral cortex. Chronic (5 days) heroin (5-30 mg/kg) and morphine (10-100 mg/kg) were also associated with increased native Fas (76% and 45%) and with different expressions of glycosylated Fas. In heroin-dependent rats, opiate withdrawal (48 h) resulted in a sustained increase in native Fas (107%) and in up-regulation of 51 kDa glycosylated Fas (51%). Acute treatments with selective delta-receptor (SNC-80, 10 mg/kg) or kappa-receptor (U 50488-H, 10 mg/kg) agonists did not alter the content of native or glycosylated Fas. Chronic pentazocine (10-80 mg/kg, 5 days), a mixed opiate drug and sigma(1) receptor agonist, decreased native (48%) and glycosylated (38-82%) Fas proteins. Similarly, the selective sigma(1) agonist (+)-SKF 10047 also decreased native Fas (37%) and the effect was blocked by the sigma(1) antagonist BD 1063. Brain dynamin was up-regulated by acute and/or chronic heroin (30-39%), morphine (47-85%), pentazocine (51%) and heroin withdrawal (74%). The main results indicate that chronic heroin/morphine treatment and heroin withdrawal are associated with up-regulation of 35 kDa native Fas (and with different expressions of glycosylated Fas), and also with concomitant increases of dynamin in rat brain.

Mu-opioid receptor desensitization: role of receptor phosphorylation, internalization, and representation

It is generally accepted that the internalization and desensitization of mu-opioid receptor (MOR) involves receptor phosphorylation and beta-arrestin recruitment. However, a mutant MOR, which is truncated after the amino acid residue Ser363 (MOR363D), was found to undergo phosphorylation-independent internalization and desensitization. As expected, MOR363D, missing the putative agonist-induced phosphorylation sites, did not exhibit detectable agonist-induced phosphorylation. MOR363D underwent slower internalization as reflected in the attenuation of membrane translocation of beta-arrestin 2 when compared with wild type MOR, but the level of receptor being internalized was similar to that of wild type MOR after 4 h of etorphine treatment. Furthermore, MOR363D was observed to desensitize faster than that of wild type MOR upon agonist activation. Surface biotinylation assay demonstrated that the wild type receptors recycled back to membrane after agonist-induced internalization, which contributed to the receptor resensitization and thus partially reversed the receptor desensitization. On the contrary, MOR363D did not recycle after internalization. Hence, MOR desensitization is controlled by the receptor internalization and the recycling of internalized receptor to cell surface in an active state. Taken together, our data indicated that receptor phosphorylation is not absolutely required in the internalization, but receptor phosphorylation and subsequent beta-arrestin recruitment play important roles in the resensitization of internalized receptors.

The intracellular trafficking of opioid receptors directed by carboxyl tail and a di-leucine motif in Neuro2A cells

The mu- and delta-opioid receptors (MOR and DOR) differ significantly in their intracellular trafficking. MORs recycle back to the cell surface upon agonist treatment, whereas most internalized DORs are targeted to lysosomes for degradation. By exchanging the carboxyl tail domains of MOR and DOR and expressing the receptor chimeras in mouse neuroblastoma Neuro2A cells, it could be demonstrated that the carboxyl tail domain is not the sole determinant in directing the intracellular trafficking in these Neuro2A cells. Deletion of the dileucine motif (Leu245-Leu246) within the third intracellular loop of DOR or the mutation of Leu245 to Met slowed the lysosomal targeting of these delta-opioid receptors. Meanwhile the mutation of Met264 to Leu increased the rate of agonist-induced receptor internalization and the lysosomal targeting of the wild type and the delta-opioid receptor carboxyl tail chimera of the mu-opioid receptor. These studies suggest interplay between a di-leucine motif and the carboxyl tail in the lysosomal targeting of the receptor.

Spatial regulation of Galphai protein signaling in clathrin-coated membrane microdomains containing GAIP

Regulators of G-protein signaling (RGS) proteins are GTPase-activating proteins (GAPs) that bind to Galpha subunits and attenuate G protein signaling, but where these events occur in the cell is not yet established. Here we investigated, by immunofluorescence labeling and deconvolution analysis, the site at which endogenous Galpha-interacting protein (GAIP) (RGS19) binds to Galphai3-YFP and its fate after activation of delta-opioid receptor (DOR). In the absence of agonist, GAIP is spatially segregated from Galphai3 and DOR in clathrin-coated domains (CCPs) of the cell membrane (PM), whereas Galphai3-YPF and DOR are located in non-clathrin-coated microdomains of the PM. Upon addition of agonist, Galphai3 partially colocalizes with GAIP in CCPs at the PM. When endocytosis is blocked by expression of a dynamin mutant [dyn(K44A)], there is a striking overlap in the distribution of DOR and Galphai3-YFP with GAIP in CCPs. Moreover, Galphai3-YFP and GAIP form a coprecipitable complex. Our results support a model whereby, after agonist addition, DOR and Galphai3 move together into CCPs where Galphai3 and GAIP meet and turn off G protein signaling. Subsequently, Galphai3 returns to non-clathrin-coated microdomains of the PM, GAIP remains stably associated with CCPs, and DOR is internalized via clathrin-coated vesicles. This constitutes a novel mechanism for regulation of Galpha signaling through spatial segregation of a GAP in clathrin-coated pits.

Neurotrophin-regulated sorting of opioid receptors in the biosynthetic pathway of neurosecretory cells

Neurotrophins modulate the endogenous opioid system, but the underlying mechanisms are poorly understood. We observed an unexpected effect of neurotrophin signaling on the membrane trafficking of recombinant opioid receptors expressed in neurosecretory cells. Epitope-tagged delta opioid receptor (DOR) and mu opioid receptor (MOR) were differentially localized between surface and internal membrane pools, respectively, when expressed in primary cultured hippocampal neurons, consistent with previous studies by others of natively expressing neurons. Selective intracellular targeting of DOR was observed in nerve growth factor (NGF)-differentiated PC12 neurosecretory cells but not in PC12 cells cultured in the absence of NGF, where both DOR and MOR were localized in the plasma membrane. Surprisingly, NGF initiated intracellular targeting of DOR in PC12 cells acutely, within 60 min after initial activation of TrkA. The NGF-induced intracellular pool of DOR originated from a late stage of the biosynthetic pathway after exit from the endoplasmic reticulum and processing of N-linked glycans in the Golgi, resulting in the accumulation in cells of a biochemically mature "reserve" pool of intracellular DOR that exhibited depolarization-dependent insertion into the plasma membrane. The C-terminal cytoplasmic tail of DOR contains a signal determining the specificity of NGF-regulated intracellular targeting. These results indicate that cloned opioid receptors are differentially targeted when expressed heterologously in neurosecretory cells, establish a model system that facilitates mechanistic study of this process, and suggest a novel function of neurotrophins in modulating the anterograde membrane trafficking of opioid receptors.

Human substance P receptor lacking the C-terminal domain remains competent to desensitize and internalize

Substance P receptor (SPR) and its naturally occurring splice-variant, lacking the C-terminal tail, are found in brain and spinal cord. Whether C-terminally truncated SPR desensitizes like full-length SPR is controversial. We used a multivaried approach to determine whether human SPR (hSPR) and a C-terminally truncated mutant, hSPRDelta325, differ in their desensitization and internalization. In HEK-293 cells expressing either hSPRDelta325 or hSPR, SP-induced desensitization of the two receptors was similar when measured by inositol triphosphate accumulation or by transient translocation of coexpressed PKCbetaII-GFP to the plasma membrane. Moreover, translocation of beta-arrestin 1 or 2-GFP (betaarr1-GFP or betaarr2-GFP) to the plasma membrane, and receptor internalization were also similar. However, hSPR and hSPRDelta325 differ in their phosphorylation and in their ability to form beta-arrestin-containing endocytic vesicles. Unlike hSPR, hSPRDelta325 is not phosphorylated to a detectable level in intact HEK293 cells, and whereas hSPR forms vesicles containing either betaarr1-GFP or betaarr2-GFP, hSPRDelta325 does not form any vesicles with betaarr1-GFP, and forms fewer vesicles with betaarr2-GFP. We conclude that truncated hSPR undergoes agonist-dependent desensitization and internalization without detectable receptor phosphorylation.

Agonist-specific down-regulation of the human delta-opioid receptor

Down-regulation of the delta-opioid receptor contributes to the development of tolerance to delta-opioid receptor agonists. The involvement of the carboxy terminus of the mouse delta-opioid receptor in peptide agonist-mediated down-regulation has been established. In the present study, we examined the down-regulation of the truncated human delta-opioid receptor by structurally distinct delta-opioid receptor agonists. Chinese hamster ovary (CHO) cells, expressing the full-length or truncated epitope-tagged human delta-opioid receptors were incubated with various delta-opioid receptor agonists (100 nM, 24 h), and membrane receptor levels were determined by [(3)H]naltrindole saturation binding. Each delta-opioid receptor agonist tested down-regulated the full-length receptor. Truncation of the carboxy terminus abolished down-regulation by all delta-opioid receptor agonists, except SNC80 ((+)-4-[(alphaR)-alpha-((2S,5R)-4-allyl-2,5-dimethyl-1-piperazinyl)-3-methoxybenzyl]N,N-diethylbenzamide). In addition, truncation of the C-terminus completely attenuated [D-Pen(2)-D-Pen(5)]enkephalin (DPDPE), but not SNC80-mediated [32P] incorporation into the protein immunoreactive with an anti-epitope-tagged antibody. These findings suggest that SNC80-mediated phosphorylation and down-regulation of the human delta-opioid receptor involves other receptor domains in addition to the carboxy terminus. Pertussis toxin treatment did not block SNC80-mediated down-regulation of the truncated Et-hDOR, indicating that the down-regulation is independent of G(i/o) protein activation and subsequent downstream signaling.

Molecular basis of differences in (-)(trans)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidiny)-cyclohexyl]benzeneacetamide-induced desensitization and phosphorylation between human and rat kappa-opioid receptors expressed in Chinese hamster ovary cells

The agonist (-)(trans)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidiny)-cyclohexyl]benzeneacetamide [(-)U50,488H] caused desensitization of the human kappa-opioid receptor (hkor) and Flag-tagged hkor (Flag-hkor) but not the rat kappa-opioid receptor (rkor) and Flag-tagged rkor (Flag-rkor) stably expressed in CHO cells as assessed by guanosine 5'-O-(3-[35S]thiotriphosphate) binding. In addition, (-)U50,488H stimulation enhanced phosphorylation of the Flag-hkor, but not Flag-rkor. (-)U50,488H-induced phosphorylation of the Flag-hkor was reduced by expression of the dominant negative mutant GRK2-K220R, demonstrating the involvement of G protein-coupled receptor kinases (GRKs). However, expression of GRK2 and arrestin-2 or GRK3 and arrestin-3 did not result in desensitization or phosphorylation of the Flag-rkor after (-)U50,488H pretreatment. To understand the molecular basis of the species differences, we constructed two Flag-tagged chimeric receptors, Flag-h/rkor and Flag-r/hkor, in which the C-terminal domains of Flag-hkor and Flag-rkor were switched. When stably expressed in CHO cells, Flag-r/hkor, but not Flag-h/rkor, was desensitized and phosphorylated after exposure to (-)U50,488H, indicating that the C-terminal domain plays a critical role in the differences. We then generated a Flag-hkor mutant, in which S358 was mutated to N (Flag-hkorS358N) and a Flag-rkor mutant, in which N358 was substituted with S (Flag-rkorN358S). Although Flag-hkorS358N was not phosphorylated or desensitized by (-)U50,488H stimulation, Flag-rkorN358S underwent (-)U50,488H-induced desensitization with slightly increased phosphorylation. These results indicate that there are differences in (-)U50,488H-induced desensitization and phosphorylation between the hkor and the rkor. In addition, the C-terminal domain plays a crucial role in these differences and the 358 locus contributes to the differences. Our findings suggest caution in extrapolating studies on kappa-opioid receptor regulation from rats to humans.

A transplantable sorting signal that is sufficient to mediate rapid recycling of G protein-coupled receptors

The beta(2)-adrenergic receptor and delta opioid receptor represent distinct G protein-coupled receptors that undergo agonist-induced endocytosis via clathrin-coated pits but differ significantly in their postendocytic sorting between recycling and degradative membrane pathways, respectively. Previous results indicate that a distal portion of the carboxyl-terminal cytoplasmic domain of the beta(2)-adrenergic receptor, which engages in PDZ domain-mediated protein interaction, is required for efficient recycling of receptors after agonist-induced endocytosis. Here we demonstrate that a four-residue sequence (DSLL) comprising the core of this protein interaction domain functions as a transplantable endocytic sorting signal that is sufficient to re-route endocytosed delta opioid receptor into a rapid recycling pathway, to inhibit proteolytic down-regulation of receptors, and to mediate receptor-autonomous sorting of mutant receptors from the wild type allele when co-expressed in the same cells. These observations define a transplantable signal mediating rapid recycling of a heterologous G protein-coupled receptor, and they suggest that rapid recycling of certain membrane proteins does not occur by bulk membrane flow but is instead mediated by a specific endocytic sorting mechanism.

Divers pathways mediate delta-opioid receptor down regulation within the same cell

Various mechanisms have been proposed for opioid receptor down regulation in different experimental preparations. The present study was aimed to test whether distinct mechanisms can mediate opioid receptor down regulation within the same cell. For this purpose we transfected HEK-293 cells with rat delta-opioid receptor (DOR). We exposed the cells to the opioid agonist etorphine in the absence or presence of various pharmacological agents and measured the binding of the opioid ligand [(3)H]diprenorphine to either isolated cell membranes or whole cells. We found that internalization of the receptors into the cell was mediated by clathrin coated pits and that the internalized receptors were degraded either in lysosomes or by proteosomes. Down regulation involved phosphorylation and at least two different kinases, a tyrosine kinase (TK) and MAPK kinase (MEK), mediated DOR down regulation in parallel routes. G-protein-coupled receptor kinase (GRK) was found to have only a minor role in DOR down regulation in HEK-293 cells. On the other hand, in N18TG2 cells that endogenously express delta-opioid receptors, GRK was the predominant kinase mediating DOR down regulation, with only a minor role for TK and MEK. We conclude that down regulation can take place via divers pathways within the same cell, and that in different cells down regulation is mediated by different mechanisms, depending on the kinase profile of the cells and the compartmentalization of the receptors within the cells.

Agonist-induced signaling, desensitization, and internalization of a phosphorylation-deficient AT1A angiotensin receptor

An analysis of the functional role of a diacidic motif (Asp236-Asp237) in the third intracellular loop of the AT1A angiotensin II (Ang II) receptor (AT1-R) revealed that substitution of both amino acids with alanine (DD-AA) or asparagine (DD-NN) residues diminished Ang II-induced receptor phosphorylation in COS-7 cells. However, Ang II-stimulated inositol phosphate production, mitogen-activated protein kinase, and AT1 receptor desensitization and internalization were not significantly impaired. Overexpression of dominant negative G protein-coupled receptor kinase 2 (GRK2)K220M decreased agonist-induced receptor phosphorylation by approximately 40%, but did not further reduce the impaired phosphorylation of DD-AA and DD-NN receptors. Inhibition of protein kinase C by bisindolylmaleimide reduced the phosphorylation of both the wild-type and the DD mutant receptors by approximately 30%. The inhibitory effects of GRK2K220M expression and protein kinase C inhibition by bisindolylmaleimide on agonist-induced phosphorylation were additive for the wild-type AT1-R, but not for the DD mutant receptor. Agonist-induced internalization of the wild-type and DD mutant receptors was similar and was unaltered by coexpression of GRK2K220M. These findings demonstrate that an acidic motif at position 236/237 in the third intracellular loop of the AT1-R is required for optimal Ang II-induced phosphorylation of its carboxyl-terminal tail by GRKs. Furthermore, the properties of the DD mutant receptor suggest that not only Ang II-induced signaling, but also receptor desensitization and internalization, are independent of agonist-induced GRK-mediated phosphorylation of the AT1 receptor.

A phosphorylation-regulated brake mechanism controls the initial endocytosis of opioid receptors but is not required for post-endocytic sorting to lysosomes

The delta-opioid receptor (DOR) can undergo proteolytic down-regulation by endocytosis of receptors followed by sorting of internalized receptors to lysosomes. Although phosphorylation of the receptor is thought to play an important role in controlling receptor down-regulation, previous studies disagree on whether phosphorylation is actually required for the agonist-induced endocytosis of opioid receptors. Furthermore, no previous studies have determined whether phosphorylation is required for subsequent sorting of internalized receptors to lysosomes. We have addressed these questions by examining the endocytic trafficking of a series of mutant versions of DOR expressed in stably transfected HEK 293 cells. Our results confirm that phosphorylation is not required for agonist-induced endocytosis of truncated mutant receptors that lack the distal carboxyl-terminal cytoplasmic domain containing sites of regulatory phosphorylation. However, phosphorylation is required for endocytosis of full-length receptors. Mutation of all serine/threonine residues located in the distal carboxyl-terminal tail domain of the full-length receptor to alanine creates functional mutant receptors that exhibit no detectable agonist-induced endocytosis. Substitution of these residues with aspartate restores the ability of mutant receptors to undergo agonist-induced endocytosis. Studies using green fluorescent protein-tagged versions of arrestin-3 suggest that the distal tail domain, when not phosphorylated, inhibits receptor-mediated recruitment of beta-arrestins to the plasma membrane. Biochemical and radioligand binding studies indicate that, after endocytosis occurs, phosphorylation-defective mutant receptors traffic to lysosomes with similar kinetics as wild type receptors. We conclude that phosphorylation controls endocytic trafficking of opioid receptors primarily by regulating a "brake" mechanism that prevents endocytosis of full-length receptors in the absence of phosphorylation. After endocytosis occurs, subsequent steps of membrane trafficking mediating sorting and transport to lysosomes do not require receptor phosphorylation.

Casein kinase II sites in the intracellular C-terminal domain of the thyrotropin-releasing hormone receptor and chimeric gonadotropin-releasing hormone receptors contribute to beta-arrestin-dependent internalization

We have previously shown that the mammalian gonadotropin-releasing hormone receptor (GnRHR), a unique G-protein-coupled receptor (GPCR) lacking an intracellular carboxyl tail (C-tail), does not follow a beta-arrestin-dependent internalization pathway. However, internalization of a chimeric GnRHR with the thyrotropin-releasing hormone receptor (TRHR) C-tail does utilize beta-arrestin. Here, we have investigated the sites within the intracellular C-tail domain that are important for conferring beta-arrestin-dependent internalization. In contrast to the chimeric GnRHR with a TRHR C-tail, a chimeric GnRHR with the catfish GnRHR C-tail is not beta-arrestin-dependent. Sequence comparisons between these chimeric receptors show three consensus phosphorylation sites for casein kinase II (CKII) in the TRHR C-tail but none in the catfish GnRHR C-tail. We thus investigated a role for CKII sites in determining GPCR internalization via beta-arrestin. Sequential introduction of three CKII sites into the chimera with the catfish C-tail (H354D,A366E,G371D) resulted in a change in the pattern of receptor phosphorylation and beta-arrestin-dependence, which only occurred when all three sites were introduced. Conversely, mutation of the putative CKII sites (T365A,T371A,S383A) in the C-tail of a beta-arrestin-sensitive GPCR, the TRHR, resulted in decreased receptor phosphorylation and a loss of beta-arrestin-dependence. Mutation of all three CKII sites was necessary before a loss of beta-arrestin-dependence was observed. Visualization of beta-arrestin/GFP redistribution confirmed a loss or gain of beta-arrestin sensitivity for receptor mutants. Internalization of receptors without C-tail CKII sites was promoted by a phosphorylation-independent beta-arrestin mutant (R169E), suggesting that these receptors do not contain the necessary phosphorylation sites required for beta-arrestin-dependent internalization. Apigenin, a specific CKII inhibitor, blocked the increase in receptor internalization by beta-arrestin, thus providing further support for the involvement of CKII. This study presents evidence of a novel role for C-tail CKII consensus sites in targeting these GPCRs to the beta-arrestin-dependent pathway.

Phosphorylation of Ser363, Thr370, and Ser375 residues within the carboxyl tail differentially regulates mu-opioid receptor internalization

Prolonged activation of opioid receptors leads to their phosphorylation, desensitization, internalization, and down-regulation. To elucidate the relationship between mu-opioid receptor (MOR) phosphorylation and the regulation of receptor activity, a series of receptor mutants was constructed in which the 12 Ser/Thr residues of the COOH-terminal portion of the receptor were substituted to Ala, either individually or in combination. All these mutant constructs were stably expressed in human embryonic kidney 293 cells and exhibited similar expression levels and ligand binding properties. Among those 12 Ser/Thr residues, Ser(363), Thr(370), and Ser(375) have been identified as phosphorylation sites. In the absence of the agonist, a basal phosphorylation of Ser(363) and Thr(370) was observed, whereas [d-Ala(2),Me-Phe(4),Gly(5)-ol]enkephalin (DAMGO)-induced receptor phosphorylation occurs at Thr(370) and Ser(375) residues. Furthermore, the role of these phosphorylation sites in regulating the internalization of MOR was investigated. The mutation of Ser(375) to Ala reduced the rate and extent of receptor internalization, whereas mutation of Ser(363) and Thr(370) to Ala accelerated MOR internalization kinetics. The present data show that the basal phosphorylation of MOR could play a role in modulating agonist-induced receptor internalization kinetics. Furthermore, even though mu-receptors and delta-opioid receptors have the same motif encompassing agonist-induced phosphorylation sites, the different agonist-induced internalization properties controlled by these sites suggest differential cellular regulation of these two receptor subtypes.

Hierarchical phosphorylation of delta-opioid receptor regulates agonist-induced receptor desensitization and internalization

Treatment of HEK293 cells expressing the delta-opioid receptor with agonist [d-Pen(2,5)]enkephalin (DPDPE) resulted in the rapid phosphorylation of the receptor. We constructed several mutants of the potential phosphorylation sites (Ser/Thr) at the carboxyl tail of the receptor in order to delineate the receptor phosphorylation sites and the agonist-induced desensitization and internalization. The Ser and Thr were substituted to alanine, and the corresponding mutants were transiently and stably expressed in HEK293 cells. We found that only two residues, i.e. Thr(358) and Ser(363), were phosphorylated, with Ser(363) being critical for the DPDPE-induced phosphorylation of the receptor. Furthermore, using alanine and aspartic acid substitutions, we found that the phosphorylation of the receptor is hierarchical, with Ser(363) as the primary phosphorylation site. Here, we demonstrated that DPDPE-induced rapid receptor desensitization, as measured by adenylyl cyclase activity, and receptor internalization are intimately related to phosphorylation of Thr(358) and Ser(363), with Thr(358) being involved in the receptor internalization.

Endomorphin-1 induced desensitization and down-regulation of the recombinant mu-opioid receptor

1. Endomorphin-1 (E1) is a peptide with high affinity and selectivity for the mu-opioid receptor. The aim of this study was to determine if endomorphin-1 caused desensitization and down-regulation of the mu-opioid receptor expressed in Chinese hamster ovary cells. 2. Following 10 microM E1 pre-treatment, desensitization was assessed by measuring cyclic AMP inhibition, down-regulation was assessed by [(3)H]-diprenorphine ([(3)H]-DPN) binding and immuno-blotting. 3. Pre-treatment of CHO mu cells with 10 microM E1 for 11 and 18 h caused significant reduction in cyclic AMP inhibition. (11 h=39.0+/-16.7%, 18 h 47.0+/-11.1% reduction). 4. At 18 h E1 pre-treatment there was an enhancement (4.5 fold) of cyclic AMP production under forskolin stimulated conditions accompanied by a small rightward shift in the concentration-response curve (pEC(50) control=7.8+/-0.3, pEC(50) E1=7.3+/-0.2) when cells were re-challenged with E1. 5. In membranes prepared from untreated and 0.5 h E1 pre-treated cells, addition of GTP gamma S produced a significant rightward shift in the concentration response curves for E1 displacement of [(3)H]-DPN (0 h K(i) control=7.86+/-0.11, GTP gamma S=7.37+/-0.15; 0.5 h K(i) control=7.92+/-0.12, GTP gamma S=7.36+/-0.08) This was not observed in membranes prepared from cells that had been treated with E1 for 18 h (18 h K(i) control=7.69+/-0. 11, GTP gamma S=7.75+/-0.08). 6. In whole cells E1 treatment caused a rapid loss of cell surface receptors such that at 0.5 h there was a 30.5+/-1.5 reduction (this was unchanged for 18 h). In crude membranes a loss of receptors was also observed using radioligand binding or immuno-blotting protocols. 7. These data show that E1 causes desensitization and down-regulation of the rat mu-opioid receptor expressed in CHO cells. However, these two responses appear temporally distinct.

Identification of G protein-coupled receptor kinase 2 phosphorylation sites responsible for agonist-stimulated delta-opioid receptor phosphorylation

Agonist-induced receptor phosphorylation is an initial step in opioid receptor desensitization, a molecular mechanism of opioid tolerance and dependence. Our previous research suggested that agonist-induced delta-opioid receptor (DOR) phosphorylation occurs at the receptor carboxyl terminal domain. The current study was carried out to identify the site of DOR phosphorylation during agonist stimulation and the kinases catalyzing this reaction. Truncation (Delta15) or substitutions (T358A, T361A, and S363G single or triple mutants) at the DOR cytoplasmic tail caused 80 to 100% loss of opioid-stimulated receptor phosphorylation, indicating that T358, T361, and S363 all contribute and are cooperatively involved in agonist-stimulated DOR phosphorylation. Coexpression of GRK2 strongly enhanced agonist-stimulated phosphorylation of the wild-type DOR (WT), but Delta15 or mutant DOR (T358A/T361A/S363G) failed to show any detectable phosphorylation under these conditions. These results demonstrate that T358, T361, and S363 are required for agonist-induced and GRK-mediated receptor phosphorylation. Agonist-induced receptor phosphorylation was severely impaired by substitution of either T358 or S363 with aspartic acid residue, but phosphorylation of the T361D mutant was comparable with that of WT. In the presence of exogenously expressed GRK2, phosphorylation levels of T358D and S363D mutants were approximately half of that of WT, whereas significant phosphorylation of the T358/S363 double-point mutant was not detected. These results indicate that both T358 and S363 residues at the DOR carboxyl terminus are capable of serving cooperatively as phosphate acceptor sites of GRK2 in vivo. Taken together, we have demonstrated that agonist-induced opioid receptor phosphorylation occurs exclusively at two phosphate acceptor sites (T358 and S363) of GRK2 at the DOR carboxyl terminus. These results represent the identification of the GRK phosphorylation site on an opioid receptor for the first time and demonstrate that GRK is the prominent kinase responsible for agonist-induced opioid receptor phosphorylation in vivo.

Internalization and sequestration of the human prostacyclin receptor

Prostacyclin (PGI(2)), the major product of cyclooxygenase in macrovascular endothelium, mediates its biological effects through its cell surface G protein-coupled receptor, the IP. PKC-mediated phosphorylation of human (h) IP is a critical determinant of agonist-induced desensitization (Smyth, E. M., Hong Li, W., and FitzGerald, G. A. (1998) J. Biol. Chem. 273, 23258-23266). The regulatory events that follow desensitization are unclear. We have examined agonist-induced sequestration of hIP. Human IP, tagged at the N terminus with hemagglutinin (HA) and fused at the C terminus to the green fluorescent protein (GFP), was coupled to increased cAMP (EC(50) = 0.39 +/- 0.09 nm) and inositol phosphate (EC(50) = 86. 6 +/- 18.3 nm) generation when overexpressed in HEK 293 cells. Iloprost-induced sequestration of HAhIP-GFP, followed in real time by confocal microscopy, was partially colocalized to clathrin-coated vesicles. Iloprost induced a time- and concentration-dependent loss of cell surface HA, indicating receptor internalization, which was prevented by inhibitors of clathrin-mediated trafficking and partially reduced by cotransfection of cells with a dynamin dominant negative mutant. Sequestration (EC(50) = 27.6 +/- 5.7 nm) was evident at those concentrations of iloprost that induce PKC-dependent desensitization. Neither the PKC inhibitor GF109203X nor mutation of Ser-328, the site for PKC phosphorylation, altered receptor sequestration indicating that, unlike desensitization, internalization is PKC-independent. Deletion of the C terminus prevented iloprost-induced internalization, demonstrating the critical nature of this region for sequestration. Internalization was unaltered by cotransfection of cells with G protein-coupled receptor kinases (GRK)-2, -3, -5, -6, arrestin-2, or an arrestin-2 dominant negative mutant, indicating that GRKs and arrestins do not play a role in hIP trafficking. The hIP is sequestered in response to agonist activation via a PKC-independent pathway that is distinct from desensitization. Trafficking is dependent on determinants located in the C terminus, is GRK/arrestin-independent, and proceeds in part via a dynamin-dependent clathrin-coated vesicular endocytotic pathway although other dynamin-independent pathways may also be involved.

Deltorphin II-induced rapid desensitization of delta-opioid receptor requires both phosphorylation and internalization of the receptor

Similar to other G protein-coupled receptors, rapid phosphorylation of the delta-opioid receptor in the presence of agonist has been reported. Hence, agonist-induced desensitization of the delta-opioid receptor has been suggested to be via the receptor phosphorylation, arrestin-mediated pathway. However, due to the highly efficient coupling between the delta-opioid receptor and the adenylyl cyclase, the direct correlation between the rates of receptor phosphorylation and receptor desensitization as measured by the adenylyl cyclase activity could not be established. In the current studies, using an ecdysone-inducible expression system to control the delta-opioid receptor levels in HEK293 cells, we could demonstrate that the rate of deltorphin II-induced receptor desensitization is dependent on the receptor level. Only at receptor concentrations </=90 fmol/mg of protein were rapid desensitizations (t(12) <10 min) observed. Apparently, deltorphin II-induced receptor desensitization involves cellular events in addition to receptor phosphorylation. Mutation of Ser(363) in the carboxyl tail of the delta-opioid receptor to Ala completely abolished the deltorphin II-induced receptor phosphorylation but not the desensitization response. Although the magnitude of desensitization was attenuated, the rate of deltorphin II-induced receptor desensitization remained the same in the S363A mutant as compared with wild type. Also, the S363A mutant could internalize in the presence of deltorphin II. Only when the agonist-induced clathrin-coated pit-mediated receptor internalization was blocked by 0.4 m sucrose that the deltorphin II-induced receptor desensitization was abolished in the S363A mutant. Similarly, 0.4 m sucrose could partially block the agonist-induced rapid desensitization in HEK293 cells expressing the wild type delta-opioid receptor. Taken together, these data supported the hypothesis that rapid desensitization of the delta-opioid receptor involves both the phosphorylation and the internalization of the receptor.

Molecular mechanisms and regulation of opioid receptor signaling

Cloning of multiple opioid receptors has presented opportunities to investigate the mechanisms of multiple opioid receptor signaling and the regulation of these signals. The subsequent identification of receptor gene structures has also provided opportunities to study the regulation of receptor gene expression and to manipulate the concentration of the gene products in vivo. Thus, in the current review, we examine recent advances in the delineation basis for the multiple opioid receptor signaling, and their regulation at multiple levels. We discuss the use of receptor knockout animals to investigate the function and the pharmacology of these multiple opioid receptors. The reasons and basis for the multiple opioid receptor are addressed.

Mechanisms and functions of AT(1) angiotensin receptor internalization

The type 1 (AT(1)) angiotensin receptor, which mediates the known physiological and pharmacological actions of angiotensin II, activates numerous intracellular signaling pathways and undergoes rapid internalization upon agonist binding. Morphological and biochemical studies have shown that agonist-induced endocytosis of the AT(1) receptor occurs via clathrin-coated pits, and is dependent on two regions in the cytoplasmic tail of the receptor. However, it is independent of G protein activation and signaling, and does not require the conserved NPXXY motif in the seventh transmembrane helix. The dependence of internalization of the AT(1) receptor on a cytoplasmic serine-threonine-rich region that is phosphorylated during agonist stimulation suggests that endocytosis is regulated by phosphorylation of the AT(1) receptor tail. beta-Arrestins have been implicated in the desensitization and endocytosis of several G protein-coupled receptors, but the exact nature of the adaptor protein required for association of the AT(1) receptor with clathrin-coated pits, and the role of dynamin in the internalization process, are still controversial. There is increasing evidence for a role of internalization in sustained signal generation from the AT(1) receptor. Several aspects of the mechanisms and specific function of AT(1) receptor internalization, including its precise mode and route of endocytosis, and the potential roles of cytoplasmic and nuclear receptors, remain to be elucidated.

Selective recruitment of arrestin-3 to clathrin coated pits upon stimulation of G protein-coupled receptors

Non-visual arrestins (arrestin-2 and arrestin-3) play critical roles in the desensitization and internalization of many G protein-coupled receptors. In vitro experiments have shown that both non-visual arrestins bind with high and approximately comparable affinities to activated, phosphorylated forms of receptors. They also exhibit high affinity binding, again of comparable magnitude, to clathrin. Further, agonist-promoted internalization of many receptors has been found to be stimulated by exogenous over-expression of either arrestin2 or arrestin3. The existence of multiple arrestins raises the question whether stimulated receptors are selective for a specific endogenous arrestin under more physiological conditions. Here we address this question in RBL-2H3 cells, a cell line that expresses comparable levels of endogenous arrestin-2 and arrestin-3. When (beta)(2)-adrenergic receptors are stably expressed in these cells the receptors internalize efficiently following agonist stimulation. However, by immunofluorescence microscopy we determine that only arrestin-3, but not arrestin-2, is rapidly recruited to clathrin coated pits upon receptor stimulation. Similarly, in RBL-2H3 cells that stably express physiological levels of m1AChR, the addition of carbachol selectively induces the localization of arrestin-3, but not arrestin-2, to coated pits. Thus, this work demonstrates coupling of G protein-coupled receptors to a specific non-visual arrestin in an in vivo setting.

Mutation of tyrosine 318 (Y318F) in the delta-opioid receptor attenuates tyrosine phosphorylation, agonist-dependent receptor internalization, and mitogen-activated protein kinase activation

Opioid receptors are known for their ability to activate diverse second messenger systems. Previously, we showed that selective delta-opioid agonists were able to induce the rapid tyrosine phosphorylation of delta-opioid receptors (delta-ORs) through Src. Src-dependent tyrosine phosphorylation of delta-ORs appears to be important for activation of the mitogen-activated protein kinase cascade and for receptor sequestration into clathrin-coated endosomes, as the Src antagonist, PP1, inhibited both. In an attempt to clarify the role of tyrosine phosphorylation in delta-OR signalling and regulation, we constructed a mutant receptor in which the tyrosine located in the conserved NPXXY motif of the C-terminus was replaced by a phenylalanine (Y318F-delta-OR). Mutation of Y318 resulted in a receptor that was comparable to the wild type in its expression level in HEK-293 cells and in its affinity for opioid ligands. Both receptors showed effective coupling to G proteins and were capable of inhibiting forskolin-stimulated cAMP accumulation with similar potencies. However, the mutant receptor was able to stimulate (35)S-GTPgammaS binding with a lower EC(50) than the wild type receptor. The stimulation of tyrosine phosphorylation in delta-ORs by [D-Thr(2)]-Leu-enkephalin-Thr (DTLET) was significantly less in cells expressing the Y318F-delta-OR than in cells expressing the wild type. In addition, both rapid receptor internalization and down-regulation were markedly attenuated in the mutant. Finally, the mutant receptor was unable to induce a robust activation of the MAPK pathway, suggesting that tyrosine phosphorylation of the delta-OR protein is important for this signalling pathway. These findings implicate tyrosine phosphorylation of Y318 in receptor signalling and agonist-mediated regulation.

Mutational analysis of the structure and function of opioid receptors

The cloning of the opioid receptors allows the investigation of receptor domains involved in the peptidic and nonpeptidic ligand interaction and activation of the opioid receptors. Receptor chimera studies and mutational analysis of the primary sequences of the opioid receptors have provided insights into the structural domains required for the ligand recognition and receptor activation. In the current review, we examine the current reports on the possible involvement of extracellular domains and transmembrane domains in the high-affinity binding of peptidic and nonpeptidic ligands to the opioid receptor. The structural requirement for the receptors' selectivity toward different ligands is discussed. The receptor domains involved in the activation and subsequent cellular regulation of the receptors' activities as determined by mutational analysis will also be discussed. Finally, the validity of the conclusions based on single amino acid mutations is examined.

Type-specific sorting of G protein-coupled receptors after endocytosis

The beta(2)-adrenergic receptor (B2AR) and delta-opioid receptor (DOR) are structurally distinct G protein-coupled receptors (GPCRs) that undergo rapid, agonist-induced internalization by clathrin-coated pits. We have observed that these receptors differ substantially in their membrane trafficking after endocytosis. B2AR expressed in stably transfected HEK293 cells exhibits negligible (<10%) down-regulation after continuous incubation of cells with agonist for 3 h, as assessed both by radioligand binding (to detect functional receptors) and immunoblotting (to detect total receptor protein). In contrast, DOR exhibits substantial (>/=50%) agonist-induced down-regulation when examined by similar means. Degradation of internalized DOR is sensitive to inhibitors of lysosomal proteolysis. Flow cytometric and surface biotinylation assays indicate that differential sorting of B2AR and DOR between distinct recycling and non-recycling pathways (respectively) can be detected within approximately 10 min after endocytosis, significantly before the onset of detectable proteolytic degradation of receptors ( approximately 60 min after endocytosis). Studies using pulsatile application of agonist suggest that after this sorting event occurs, later steps of membrane transport leading to lysosomal degradation of receptors do not require the continued presence of agonist in the culture medium. These observations establish that distinct GPCRs differ significantly in endocytic membrane trafficking after internalization by the same membrane mechanism, and they suggest a mechanism by which brief application of agonist can induce substantial down-regulation of receptors.