Opioids and their complicated receptor complexes

No field more eagerly awaits a molecular clarification for G-protein coupled receptor (GPCR) dimerization than the opioid receptor field. Extensive evidence of pharmacological and functional interactions between opioid receptor types has primed this field for such a resolution. In retrospect, much of the data collected on synergy between different opioid receptor types may represent the functional correlate for the newly found opioid receptor dimerization. While previous reports of functional synergy have been, for the most part, consistent in demonstrating cross-regulation between two receptor types, the lack of highly receptor-selective ligands allowed skeptics to remain doubtful over the interpretations of these results. Today, two important developments in the opioid receptor field help reinvigorate the hypothesis of functional, cross-modulating opioid receptor complexes: (1) The existence of highly selective ligands which eliminate any possibility of cross-reactivity between receptor types, and (2) the discovery that opioid receptors and a number of other GPCRs exist as dimers in biochemical, functional and pharmacological assays. It is with these new tools that we seek to understand the mechanisms and implications of dimerization. Initial results of these studies have demonstrated that the dimerization of opioid receptors may help consolidate several pharmacological findings that have remained unanswered. In this review we present biochemical, pharmacological and functional evidence for opioid receptor complexes and add evidence from our recent studies on opioid receptor dimerization. We believe a thorough understanding of receptor dimerization is crucial in clarifying the mechanism of action of opioids and other drugs and may serve a more practical purpose in aiding the development of novel therapeutic drugs.

Signaling of G(alpha)(12) family of G proteins through a tyrosine kinase and a Ras-GAP

G(12) heterotrimeric G proteins transduce signals from a number of receptors involved in the regulation of cardiovascular function. Recently, it has been discovered that these G proteins regulate the activity of Bruton's tyrosine kinase and Ras-GAP1(m) through a conserved PH-BM domain. These findings provide new insights into the signaling pathways initiated by G(12), which regulate cytoskeletal organization, cell proliferation and cardiovascular function.

Opioid receptor endocytosis and activation of MAP kinase pathway

Opioid receptors, members of the G-protein coupled receptor (GPCR) super family, bind to endogenous opioid peptides or opiate drugs and induce a wide variety of signal transduction processes by inhibiting adenylyl cyclase, modulating cation channels, and activating the mitogen-activated protein (MAP) kinases. Similar to other GPCRs, agonist binding causes rapid internalization and down-regulation of opioid receptors. The interdependence between receptor endocytosis and activation of MAP kinase pathway are increasingly being examined. We have examined these using ligands that exhibit differential extent of endocytosis as well as mutants of mu and delta opioid receptors that are unable to internalize. We find that ligands, including morphine, that do not induce receptor internalization are able to stimulate MAP kinase phosphorylation not only in heterologous cells but also in neuronal cell lines that express endogenous mu and delta receptors. Moreover, mutant receptors that fail to undergo agonist-mediated internalization are able to efficiently phosphorylate MAP kinases. Taken together, these data are consistent with the notion that the activation of MAP kinase pathway is an internalization independent phenomenon in the case of opioid receptors and that GPCR internalization and activation of MAP kinase are governed by complex regulatory mechanisms.

Role for C-tail residues in delta opioid receptor downregulation

The delta opioid receptor, a member of the G-protein-coupled receptor superfamily, was used as a model system to characterize opioid receptor downregulation. Metabolic labeling followed by immunoprecipitation resulted in the isolation of the epitope-tagged mouse delta opioid receptor as a approximately 60-kDa protein. Prolonged agonist treatment with 100 nM d-Ala2, d-Leu5-enkephalin (DADLE) caused significant (approximately 60%) reduction in the level of receptor. The delta opioid receptor contains a number of phosphorylatable residues in the C tail. Point mutations of the majority of Ser/Thr sequences did not affect the level of downregulation, whereas mutation of Thr353 to Ala did. In order to test if phosphorylation at this site is involved in receptor downregulation, we generated a Thr353Glu mutant that would mimic the phosphorylated Thr at this site. This mutant exhibited a significantly higher extent of downregulation than the Thr353Ala mutant. In order to critically evaluate the requirement of Thr353 in receptor downregulation, we examined the downregulation of wildtype rat delta receptor (which does not contain Ala353) and an Ala353Thr point-mutant rat delta receptor. The wild-type receptor exhibited poor agonist-mediated downregulation, whereas Ala353Thr mutant exhibited increased downregulation. These results and results from additional studies with rat/mouse chimeric receptors support a role for phosphorylation of sites within the C tail in efficient downregulation of delta opioid receptors.

Kappa opioid receptor endocytosis by dynorphin peptides

Internalization and downregulation are important steps in the modulation of receptor function. Recent work with the beta2 adrenergic and opioid receptors have implicated these processes in receptor-mediated activation of mitogen-activated protein kinase (MAPK). We have used CHO cells expressing epitope-tagged rat kappa opioid receptors (rKORs) and prodynorphin-derived peptides to characterize the agonist-mediated endocytosis of rKORs and activation of MAPK. Kappa receptor-selective peptides induced receptor internalization and downregulation whereas nonpeptide agonists did not. An examination of the ability of dynorphin A-17-related peptides (lacking C-terminal amino acids) to promote KOR internalization, inhibition of adenylyl cyclase, and MAPK phosphorylation revealed that the N-terminal seven residues play an important role in eliciting these responses. Both dynorphin peptides and nonpeptide agonists induced rapid and robust phosphorylation of MAPKs. Taken together, these results point to a difference in the ability of dynorphin peptides and nonpeptide ligands to promote rKOR endocytosis and support the view that rKOR internalization is not required for MAPK activation.

Dimerization of the delta opioid receptor: implication for a role in receptor internalization

Dimerization of G-protein-coupled receptors has been increasingly noted in the regulation of their biological activity. However, its involvement in agonist-induced receptor internalization is not well understood. In this study, we examined the ability of mouse delta-opioid receptors to dimerize and the role of receptor dimerization in agonist-induced internalization. Using differentially (Flag and c-Myc) epitope-tagged receptors we show that delta-opioid receptors exist as dimers. The level of dimerization is agonist dependent. Increasing concentrations of agonists reduce the levels of dimer with a corresponding increase in the levels of monomer. Interestingly, morphine does not affect the levels of either form. It has been shown that morphine, unlike other opioid agonists, does not induce receptor internalization. This suggests a relationship between the ability of agonists to reduce the levels of dimer and to induce receptor internalization. The time course of the agonist-induced decrease of delta-opioid receptor dimers is shorter than the time course of internalization, suggesting that monomerization precedes the agonist-induced internalization of the receptor. Furthermore, we found that a mutant delta-opioid receptor, with a 15-residue C-terminal deletion, does not exhibit dimerization. This mutant receptor has been shown to lack the ability to undergo agonist-induced internalization. These results suggest that the interconversion between the dimeric and monomeric forms plays a role in opioid receptor internalization.

Sequestration of the delta opioid receptor. Role of the C terminus in agonist-mediated internalization

The primary structure of the opioid receptors have revealed that many of the structural features that are conserved in other G protein-coupled receptors are also conserved in the opioid receptors. Upon exposure to agonists, some G protein-coupled receptors internalize rapidly, whereas other structurally homologous G protein-coupled receptors do not. It is not known whether opioid receptors are regulated by rapid endocytosis. In transfected Chinese hamster ovary cells expressing the epitope-tagged wild type delta opioid receptor, exposure to 100 nM [D-Ala2,D-Leu5]enkephalin causes internalization of the receptor within 30 min as determined by confocal microscopy. The rate of internalization of the wild type receptor is rapid with a half-maximal reduction by about 10 min, as determined by the reduction in mean surface receptor fluorescence intensity measured using flow cytometry. In contrast, the cells expressing receptors lacking the C-terminal 15 or 37 amino acids exhibit a substantially slower rate of internalization. Furthermore, the cells expressing receptors with point mutations of any of the Ser/Thr between Ser344 and Ser363 in the C-terminal tail exhibit a significant reduction in the rate of receptor internalization. These results suggest that a portion of the C-terminal tail is involved in receptor internalization. Agents that block the formation of clathrin-coated pits considerably reduce the extent of agonist-mediated internalization of the wild type receptor. Taken together, these results suggest that the wild type opioid receptor undergoes rapid agonist-mediated internalization via a classic endocytic pathway and that a portion of the C-terminal tail plays an important role in this internalization process.

Thr353, located within the COOH-terminal tail of the delta opiate receptor, is involved in receptor down-regulation

Prolonged exposure to abused drugs such as opiates causes decreased response to the drug; this reduced sensitivity is thought to be due to the loss of receptors, or down-regulation. The molecular mechanism of the opiate receptor down-regulation is not known. In order to address this, we generated a number of mutants of the delta opiate receptor COOH-terminal tail. When expressed in the Chinese hamster ovary cells, both the wild type and the receptor with a deletion of 37 COOH-terminal residues bind diprenorphine with comparable affinities and show similar decreases in cAMP levels in response to D-Ala2, D-Leu5, enkephalin (DADLE). However, the truncated receptor does not show down-regulation from the cell surface upon prolonged exposure (2-48 h) to DADLE. In contrast, both the wild type receptor and the receptor with the deletion of only 15 COOH-terminal residues show substantial down-regulation upon long term DADLE treatment. These results suggest that the region located between 15 and 37 residues from the COOH terminus is involved in the receptor down-regulation. In order to identify residues that play a key role in down-regulation, point mutations of residues within this region were examined for their ability to modulate receptor down-regulation. The receptor with a mutation of Thr353 to Ala does not down-regulate, whereas the receptor with a mutation of Ser344 to Gly down-regulates with a time course similar to that of the wild type receptor. Taken together, these results suggest that the COOH-terminal tail is not essential for functional coupling but is necessary for down-regulation and that Thr353 is critical for the agonist-mediated down-regulation of the delta opiate receptor.