Agonists activate Gi1 alpha or Gi2 alpha fused to the human mu opioid receptor differently

Fusion with Promiscuous Gα16 Subunit Reveals Signaling Bias at Muscarinic Receptors

A complex evaluation of agonist bias at G-protein coupled receptors at the level of G-protein classes and isoforms including non-preferential ones is essential for advanced agonist screening and drug development. Molecular crosstalk in downstream signaling and a lack of sufficiently sensitive and selective methods to study direct coupling with G-protein of interest complicates this analysis. We performed binding and functional analysis of 11 structurally different agonists on prepared fusion proteins of individual subtypes of muscarinic receptors and non-canonical promiscuous α-subunit of G₁₆ protein to study agonist bias. We have demonstrated that fusion of muscarinic receptors with Gα₁₆ limits access of other competitive Gα subunits to the receptor, and thus enables us to study activation of Gα₁₆ mediated pathway more specifically. Our data demonstrated agonist-specific activation of G₁₆ pathway among individual subtypes of muscarinic receptors and revealed signaling bias of oxotremorine towards Gα₁₆ pathway at the M₂ receptor and at the same time impaired Gα₁₆ signaling of iperoxo at M₅ receptors. Our data have shown that fusion proteins of muscarinic receptors with α-subunit of G-proteins can serve as a suitable tool for studying agonist bias, especially at non-preferential pathways.

Mice Expressing Regulators of G protein Signaling-insensitive Gαo Define Roles of μ Opioid Receptor Gαo and Gαi Subunit Coupling in Inhibition of Presynaptic GABA Release

Regulators of G protein signaling (RGS) proteins modulate signaling by G protein-coupled receptors. Using a knock-in transgenic mouse model with a mutation in Gαo that does not bind RGS proteins (RGS-insensitive), we determined the effect of RGS proteins on presynaptic μ opioid receptor (MOR)-mediated inhibition of GABA release in the ventrolateral periaqueductal gray (vlPAG). The MOR agonists [d-Ala², N-MePhe⁴, Gly-ol]-enkephalin (DAMGO) and met-enkephalin (ME) inhibited evoked inhibitory postsynaptic currents (eIPSCs) in the RGS-insensitive mice compared with wild-type (WT) littermates, respectively. Fentanyl inhibited eIPSCs similarly in both WT and RGS-insensitive mice. There were no differences in opioid agonist inhibition of spontaneous GABA release between the genotypes. To further probe the mechanism underlying these differences between opioid inhibition of evoked and spontaneous GABA release, specific myristoylated Gα peptide inhibitors for Gαo₁ and Gαi_1-3 that block receptor-G protein interactions were used to test the preference of agonists for MOR-Gα complexes. The Gαo₁ inhibitor reduced DAMGO inhibition of eIPSCs, but Gαi_1-3 inhibitors had no effect. Both Gαo₁ and Gαi_1-3 inhibitors separately reduced fentanyl inhibition of eIPSCs but had no effects on ME inhibition. Gαi_1-3 inhibitors blocked the inhibitory effects of ME and fentanyl on miniature postsynaptic current (mIPSC) frequency, but both Gαo₁ and Gαi_1-3 inhibitors were needed to block the effects of DAMGO. Finally, baclofen-mediated inhibition of GABA release is unaffected in the RGS-insensitive mice and in the presence of Gαo₁ and Gαi_1-3 inhibitor peptides, suggesting that GABA_B receptor coupling to G proteins in vlPAG presynaptic terminals is different than MOR coupling. SIGNIFICANCE STATEMENT: Presynaptic μ opioid receptors (MORs) in the ventrolateral periaqueductal gray are critical for opioid analgesia and are negatively regulated by RGS proteins. These data in RGS-insensitive mice provide evidence that MOR agonists differ in preference for Gαo versus Gαi and regulation by RGS proteins in presynaptic terminals, providing a mechanism for functional selectivity between agonists. The results further define important differences in MOR and GABA_B receptor coupling to G proteins that could be exploited for new pain therapies.

Endocrine Gland-Derived Vascular Endothelial Growth Factor/Prokineticin-1 in Cancer Development and Tumor Angiogenesis

A lot of data suggests endocrine gland-derived vascular endothelial growth factor (EG-VEGF) to be restricted to endocrine glands and to some endocrine-dependent organs. Many evidences show that EG-VEGF stimulates angiogenesis and cell proliferation, although it is not a member of the VEGF family. At the time, a lot of data regarding the role of this growth factor in normal development are available. However, controversial results have been published in the case of pathological conditions and particularly in malignant tumors. Thus, our present paper has been focused on the role of EG-VEGF in normal tissues and various malignant tumors and their angiogenic processes.

Systematic analysis of factors influencing observations of biased agonism at the mu-opioid receptor

Biased agonism describes the ability of distinct G protein-coupled receptor (GPCR) ligands to stabilise distinct receptor conformations leading to the activation of different cell signalling pathways that can deliver different physiologic outcomes. This phenomenon is having a major impact on modern drug discovery as it offers the potential to design ligands that selectively activate or inhibit the signalling pathways linked to therapeutic effects with minimal activation or blockade of signalling pathways that are linked to the development of adverse on-target effects. However, the explosion in studies of biased agonism at multiple GPCR families in recombinant cell lines has revealed a high degree of variability on descriptions of biased ligands at the same GPCR and raised the question of whether biased agonism is a fixed attribute of a ligand in all cell types. The current study addresses this question at the mu-opioid receptor (MOP). Here, we have systematically assessed the impact of differential cellular protein complement (and cellular background), signalling kinetics and receptor species on our previous descriptions of biased agonism at MOP by several opioid peptides and synthetic opioids. Our results show that all these factors need to be carefully determined and reported when considering biased agonism. Nevertheless, our studies also show that, despite changes in overall signalling profiles, ligands that previously showed distinct bias profiles at MOP retained their uniqueness across different cell backgrounds.

Biased Agonism of Endogenous Opioid Peptides at the μ-Opioid Receptor

Biased agonism is having a major impact on modern drug discovery, and describes the ability of distinct G protein-coupled receptor (GPCR) ligands to activate different cell signaling pathways, and to result in different physiologic outcomes. To date, most studies of biased agonism have focused on synthetic molecules targeting various GPCRs; however, many of these receptors have multiple endogenous ligands, suggesting that "natural" bias may be an unappreciated feature of these GPCRs. The μ-opioid receptor (MOP) is activated by numerous endogenous opioid peptides, remains an attractive therapeutic target for the treatment of pain, and exhibits biased agonism in response to synthetic opiates. The aim of this study was to rigorously assess the potential for biased agonism in the actions of endogenous opioids at the MOP in a common cellular background, and compare these to the effects of the agonist d-Ala2-N-MePhe4-Gly-ol enkephalin (DAMGO). We investigated activation of G proteins, inhibition of cAMP production, extracellular signal-regulated kinase 1 and 2 phosphorylation, β-arrestin 1/2 recruitment, and MOP trafficking, and applied a novel analytical method to quantify biased agonism. Although many endogenous opioids displayed signaling profiles similar to that of DAMGO, α-neoendorphin, Met-enkephalin-Arg-Phe, and the putatively endogenous peptide endomorphin-1 displayed particularly distinct bias profiles. These may represent examples of natural bias if it can be shown that they have different signaling properties and physiologic effects in vivo compared with other endogenous opioids. Understanding how endogenous opioids control physiologic processes through biased agonism can reveal vital information required to enable the design of biased opioids with improved pharmacological profiles and treat diseases involving dysfunction of the endogenous opioid system.

Cellular signalling of non-synonymous single-nucleotide polymorphisms of the human μ-opioid receptor (OPRM1)

There is significant variability in individual responses to opioid drugs, which is likely to have a significant genetic component. A number of non-synonymous single-nucleotide polymorphisms (SNPs) in the coding regions of the μ-opioid receptor gene (OPRM1) have been postulated to contribute to this variability. Although many studies have investigated the clinical influences of these μ-opioid receptor variants, the outcomes are reported in the context of thousands of other genes and environmental factors, and we are no closer to being able to predict individual response to opioids based on genotype. Investigation of how μ-opioid receptor SNPs affect their expression, coupling to second messengers, desensitization and regulation is necessary to understand how subtle changes in receptor structure can impact individual responses to opioids. To date, the few functional studies that have investigated the consequences of SNPs on the signalling profile of the μ-opioid receptor in vitro have shown that the common N40D variant has altered functional responses to some opioids, while other, rarer, variants display altered signalling or agonist-dependent regulation. Here, we review the data available on the effects of μ-opioid receptor polymorphisms on receptor function, expression and regulation in vitro, and discuss the limitations of the studies to date. Whether or not μ-opioid receptor SNPs contribute to individual variability in opioid responses remains an open question, in large part because we have relatively little good data about how the amino acid changes affect μ-opioid receptor function.

LINKED ARTICLES

This article is part of a themed section on Opioids: New Pathways to Functional Selectivity. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2015.172.issue-2.

Novel GPCR paradigms at the μ-opioid receptor

Opioids, such as morphine, are the most clinically useful class of analgesic drugs for the treatment of acute and chronic pain. However, the use of opioids is greatly limited by the development of severe adverse side effects. Consequently, drug discovery efforts have been directed towards improving the therapeutic profile of opioid-based treatments. Opioid receptors are members of the family of GPCRs. As such, the recent GPCR paradigms of biased agonism and allosterism may provide novel avenues for more effective analgesics. Biased agonism (or functional selectivity) has been described for all the opioid receptor family members. Furthermore, the first allosteric modulators of opioid receptors have very recently been described. However, identification and quantification of biased agonism in a manner that is informative to medicinal chemists and drug discovery programmes still remains a challenge. In this review, we examine the progress, to date, towards identification and quantification of biased agonism and allosterism at the μ-opioid receptor in the context of its implications for the discovery of better and safer analgesics.

LINKED ARTICLES

This article is part of a themed section on Opioids: New Pathways to Functional Selectivity. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2015.172.issue-2.

Probing heterotrimeric G protein activation: applications to biased ligands

Cell surface G protein-coupled receptors (GPCRs) drive numerous signaling pathways involved in the regulation of a broad range of physiologic processes. Today, they represent the largest target for modern drugs development with potential application in all clinical fields. Recently, the concept of "ligand-directed trafficking" has led to a conceptual revolution in pharmacological theory, thus opening new avenues for drug discovery. Accordingly, GPCRs do not function as simple on-off switch but rather as filters capable of selecting the activation of specific signals and thus generating texture responses to ligands, a phenomenon often referred to as ligand-biased signaling. Also, one challenging task today remains optimization of pharmacological assays with increased sensitivity so to better appreciate the inherent texture of ligands. However, considering that a single receptor has pleiotropic signaling properties and that each signal can crosstalk at different levels, biased activity remains thus difficult to evaluate. One strategy to overcome these limitations would be examining the initial steps following receptor activation. Even, if some G protein independent functions have been recently described, heterotrimeric G protein activation remains a general hallmark for all GPCRs families and the first cellular event subsequent to agonist binding to the receptor. Herein, we review the different methodologies classically used or recently developed to monitor G protein activation and discussed them in the context of G protein biased-ligands.

Functional selectivity at the μ-opioid receptor: implications for understanding opioid analgesia and tolerance

Opioids are the most effective analgesic drugs for the management of moderate or severe pain, yet their clinical use is often limited because of the onset of adverse side effects. Drugs in this class produce most of their physiological effects through activation of the μ opioid receptor; however, an increasing number of studies demonstrate that different opioids, while presumably acting at this single receptor, can activate distinct downstream responses, a phenomenon termed functional selectivity. Functional selectivity of receptor-mediated events can manifest as a function of the drug used, the cellular or neuronal environment examined, or the signaling or behavioral measure recorded. This review summarizes both in vitro and in vivo work demonstrating functional selectivity at the μ opioid receptor in terms of G protein coupling, receptor phosphorylation, interactions with β-arrestins, receptor desensitization, internalization and signaling, and details on how these differences may relate to the progression of analgesic tolerance after their extended use.

The influence of monovalent cations on trimeric G protein G(i)1α activity in HEK293 cells stably expressing DOR-G(i)1α (Cys(351)-Ile(351)) fusion protein

The effect of monovalent cations on trimeric G protein G(i)1α was measured at equimolar concentration of chloride anion in pertussis-toxin (PTX)-treated HEK293 cells stably expressing PTX-insensitive DOR- G(i)1α (Cys(351)-Ile(351)) fusion protein by high-affinity [(35)S]GTPgammaS binding assay. The high basal level of binding was detected in absence of DOR agonist and monovalent ions and this high level was inhibited with the order of: Na(+) > K(+) > Li(+). The first significant inhibition was detected at 1 mM NaCl. The inhibition by monovalent ions was reversed by increasing concentrations of DOR agonist DADLE. The maximum DADLE response was also highest for sodium and decreased in the order of: Na(+) > K(+) ~ Li(+). Our data indicate i) an inherently high activity of trimeric G protein G(i)1α when expressed within DOR- G(i)1α fusion protein and determined in the absence of monovalent cations, ii) preferential sensitivity of DOR- G(i)1alpha to sodium as far as maximum of agonist response is involved.

Molecular characterization of EG-VEGF-mediated angiogenesis: differential effects on microvascular and macrovascular endothelial cells

Endocrine gland derived vascular endothelial growth factor (EG-VEGF) also called prokineticin (PK1), has been identified and linked to several biological processes including angiogenesis. EG-VEGF is abundantly expressed in the highest vascularized organ, the human placenta. Here we characterized its angiogenic effect using different experimental procedures. Immunohistochemistry was used to localize EG-VEGF receptors (PROKR1 and PROKR2) in placental and umbilical cord tissue. Primary microvascular placental endothelial cell (HPEC) and umbilical vein-derived macrovascular EC (HUVEC) were used to assess its effects on proliferation, migration, cell survival, pseudovascular organization, spheroid sprouting, permeability and paracellular transport. siRNA and neutralizing antibody strategies were used to differentiate PROKR1- from PROKR2-mediated effects. Our results show that 1) HPEC and HUVEC express both types of receptors 2) EG-VEGF stimulates HPEC's proliferation, migration and survival, but increases only survival in HUVECs. and 3) EG-VEGF was more potent than VEGF in stimulating HPEC sprout formation, pseudovascular organization, and it significantly increases HPEC permeability and paracellular transport. More importantly, we demonstrated that PROKR1 mediates EG-VEGF angiogenic effects, whereas PROKR2 mediates cellular permeability. Altogether, these data characterized angiogenic processes mediated by EG-VEGF, depicted a new angiogenic factor in the placenta, and suggest a novel view of the regulation of angiogenesis in placental pathologies.

High-level production and characterization of a G-protein coupled receptor signaling complex

Elucidation of the molecular details of signal transduction through G-protein coupled receptors (GPCRs) awaits the solution of high-resolution structures of the receptor species involved in passing the extracellular information across the plasma membrane. The critical challenge in this effort is the production of sufficient quantities of active and homogeneous receptor species amenable to crystallization screening. We describe here the high-level expression in mammalian cells and characterization of a fusion complex between the kappa opioid receptor and its cognate G-protein alpha subunit, G alpha(i1). Optimization of growth conditions resulted in the highest level of active binding sites reported to date for either opioid receptors or GPCR-G alpha fusions. In cells, the kappa opioid receptor was stabilized against proteolysis in the context of the fusion protein and was competent to bind both agonists and antagonists. Coupling of the kappa opioid receptor with the G alpha subunit was demonstrated by changes in agonist affinity in the presence of guanine nucleotides and by agonist-induced increases in the rate of guanine nucleotide hydrolysis. In addition to representing a physiologically relevant signaling complex, the additional hydrophilic surface area provided by the G-protein may enhance the chances of producing well-diffracting crystals from the purified complex.

Functional coupling of mu-receptor-Galphai-tethered proteins in AtT20 cells

Opioid efficacy on mu-receptor may be influenced by various Gi/o-G-protein subunits interacting with intracellular face of receptor. Pertussis toxin-insensitive Galphai1 and Galphai2 subunits tethered with mu-receptor were stably transfected into AtT20 cells to (i) determine coupling of different alpha-subunits on opioid efficacy, and (ii) determine coupling to downstream effectors, for example, calcium and potassium channels. After pertussis toxin, stimulation of [35S]GTP-gamma-S incorporation persisted. Both constructs were able to couple to native calcium and potassium channels, with endomorphins 1 and 2 equally effective. However, pertussis toxin abolished opioid actions on calcium and potassium channels suggesting strong coupling to endogenous G-proteins, and that differences in coupling efficacy to Galphai1 and Galphai2 previously observed are restricted to initial step of signaling cascade.

Activation of G proteins and extracellular signal-regulated kinase 1/2 phosphorylation via human dopamine D4.4 receptors: differential pathway-dependent potencies of receptor agonists

Agonist activity at recombinant human dopamine D4.4 receptors was compared in stably transfected CHO cells using two functional readouts: G protein activation by [35S]GTPgammaS binding and phosphorylation of extracellular signal-regulated kinase 1/2 (pERK1/2). Results with a large series of agonists reveal markedly higher relative agonist efficacy in the pERK1/2 assay compared with [35S]GTPgammaS binding, while potencies were generally higher in the latter readout. Whereas efficacies were highly correlated when comparing both tests, potencies determined using the pERK1/2 assay were neither correlated with those for G protein activation nor with binding affinities. In order to examine if these differences may be attributable to distinct assay conditions (5 min incubation for pERK1/2 compared with binding equilibrium conditions for [35S]GTPgammaS), selected compounds were tested in a modified short-duration [35S]GTPgammaS binding assay. In these experiments, potencies were generally reduced; however, compounds exhibiting comparably high potency in the pERK1/2 assay were not affected by this duration-dependent potency shift. We conclude that assay parameters such as signal amplification and incubation time have to be considered with respect to the appropriate choice of experimental approaches that best reflect agonist activity at dopamine D4 receptors in vivo.

Agonist-selective signaling is determined by the receptor location within the membrane domains

The basis for agonist-selective signaling was investigated by using the mu-opioid receptor (MOR) as a model. In the absence of agonist, MOR located within the lipid raft domains, whereas etorphine, but not morphine, induced the translocation of MOR from lipid raft to nonraft domains, similar to the action of methyl-beta-cyclodextrin. The etorphine-induced MOR translocation required the dissociation of the receptor from Galphai2 first and then the binding of beta-arrestin. In contrast, the low affinity of the morphine-MOR complex for beta-arrestin and the rebinding of Galphai2 after GTP hydrolysis retained the complex within the lipid raft domains. Disruption of the MOR-Galphai2 interaction, either by deleting the (276)RRITR(280) sequence of MOR or knocking down the level of Galphai2, resulted in the translocation of MOR to the nonraft domains. In addition, lipid raft location of MOR was critical for G protein-dependent signaling, such as etorphine- and morphine-mediated inhibition of adenylyl cyclase activity and morphine-induced ERK phosphorylation, whereas beta-arrestin-dependent, etorphine-induced ERK phosphorylation required MOR to translocate into the nonraft domains. Thus, agonist-selective signaling is regulated by the location of MOR, which is determined by interactions of MOR with G proteins and beta-arrestin.

Co-expression of mu and delta opioid receptors as receptor-G protein fusions enhances both mu and delta signalling via distinct mechanisms

Mu and delta opioid receptors (MORs and DORs) were co-expressed as fusion proteins between a receptor and a pertussis insensitive mutant Galpha(i/o) protein in human embryonic kidney 293 cells. Signalling efficiency was then monitored following inactivation of endogenous Galpha(i/o) proteins by pertussis toxin. Co-expression resulted in increased delta opioid signalling which was insensitive to the mu specific antagonist d-Phe-Cys-Tyr-d-Trp-Arg-Thr-Pen-Thr-NH2. Under these conditions, mu opioid signalling was also increased and insensitive to the delta specific antagonist Tic-deltorphin. In this latter case, however, no G protein activation was observed in the presence of the delta specific inverse agonist N,N(CH3)2-Dmt-Tic-NH2. When a MOR fused to a non-functional Galpha subunit was co-expressed with the DOR-Galpha protein fusion, delta opioid signalling was not affected whereas mu opioid signalling was restored. Altogether our results suggest that increased delta opioid signalling is due to enhanced DOR coupling to its tethered Galpha subunit. On the other hand, our data indicate that increased mu opioid signalling requires an active conformation of the DOR and also results in activation of the Galpha subunit fused the DOR.

Endogenous RGS proteins attenuate Galpha(i)-mediated lysophosphatidic acid signaling pathways in ovarian cancer cells

Lysophosphatidic acid is a bioactive phospholipid that is produced by and stimulates ovarian cancer cells, promoting proliferation, migration, invasion, and survival. Effects of LPA are mediated by cell surface G-protein coupled receptors (GPCRs) that activate multiple heterotrimeric G-proteins. G-proteins are deactivated by Regulator of G-protein Signaling (RGS) proteins. This led us to hypothesize that RGS proteins may regulate G-protein signaling pathways initiated by LPA in ovarian cancer cells. To determine the effect of endogenous RGS proteins on LPA signaling in ovarian cancer cells, we compared LPA activity in SKOV-3 ovarian cancer cells expressing G(i) subunit constructs that are either insensitive to RGS protein regulation (RGSi) or their RGS wild-type (RGSwt) counterparts. Both forms of the G-protein contained a point mutation rendering them insensitive to inhibition with pertussis toxin, and cells were treated with pertussis toxin prior to experiments to eliminate endogenous G(i/o) signaling. The potency and efficacy of LPA-mediated inhibition of forskolin-stimulated adenylyl cyclase activity was enhanced in cells expressing RGSi G(i) proteins as compared to RGSwt G(i). We further showed that LPA signaling that is subject to RGS regulation terminates much faster than signaling thru RGS insensitive G-proteins. Finally, LPA-stimulated SKOV-3 cell migration, as measured in a wound-induced migration assay, was enhanced in cells expressing Galpha(i2) RGSi as compared to cells expressing Galpha(i2) RGSwt, suggesting that endogenous RGS proteins in ovarian cancer cells normally attenuate this LPA effect. These data establish RGS proteins as novel regulators of LPA signaling in ovarian cancer cells.

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.

Mu-delta opioid receptor functional interaction: Insight using receptor-G protein fusions

Fusion proteins between a receptor and a pertussis toxin-insensitive G(i)alpha subunit were used to gain insight into the molecular interactions that take place upon mu and delta opioid receptor heterodimerization. When mu opioid receptor-G(i1)alpha fusions were coexpressed with nonfused delta opioid receptors in human embryonic kidney 293 cells, or vice versa, receptor heterodimers were detected by coimmunoprecipitation. In pertussis toxin-treated cells, receptor coexpression decreased the amount of guanosine 5'-O-(3-[35S]thio)triphosphate ([35S]GTPgammaS) incorporated in the fused G alpha protein after the addition of agonists specific for the receptor-G(i1)alpha fusion. In addition, activation of the G alpha protein occurred in heterodimers upon addition of an agonist specific for the nonfused receptor. It remained unaffected by an inverse agonist specific for the receptor-G(i1)alpha fusion. These data suggest that signaling through the receptor-G(i1)alpha fusion protein is impaired in heterodimers and support a mechanism in which activation of the G alpha subunit is promoted by a direct interaction with the nonfused receptor. Alternatively, receptor coexpression did not modify the ligand binding properties for the high-affinity state of the receptor-G(i1)alpha fusion nor the EC50 values for agonist-induced [35S]GTPgammaS incorporation in the G(i1)alpha subunit. In addition, no binding competition was observed between delta and mu ligands. Together, the data point to mu-delta opioid receptor heterodimers formed by contact interactions between monomers that retain their structural integrity.

Inhibition by endomorphin-1 and endomorphin-2 of excitatory transmission in adult rat substantia gelatinosa neurons

Intrathecally-administered endomorphin-1 and endomorphin-2 produce antinociceptive effects which are different from each other. In order to elucidate a cellular basis for this result, we examined the effects of endomorphin-1 and endomorphin-2 on holding currents and spontaneous glutamatergic excitatory transmission in substantia gelatinosa neurons of adult rat spinal cord slices by use of the whole-cell patch-clamp technique. In about half of the neurons examined, endomorphin-1 and endomorphin-2 produced an outward current having a similar amplitude (25-27 pA at 1 microM) at -70 mV with almost the same value of effective concentration producing half-maximal response (0.19-0.21 microM). Both of them reversed at a potential close to the equilibrium potential for K+, and had the slope conductance that was larger at negative (-120 to -140 mV) than positive potentials (-60 to -90 mV). The endomorphin-1 and endomorphin-2 currents were reduced in amplitude by K+-channel inhibitors, Ba2+ (100 microM) and 4-aminopyridine (1 mM), and also by mu-opioid receptor antagonist D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2 (1 microM) to a similar extent. The endomorphin-2 but not endomorphin-1 current amplitude was increased by dipeptidyl peptidase IV inhibitor diprotin A (30 microM). One micromolar endomorphin-1 and endomorphin-2 reduced the frequency of spontaneous excitatory postsynaptic current with a similar time course and extent without altering its amplitude; these actions were not in the presence of D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2 (1 microM). We conclude that endomorphin-1 and endomorphin-2 hyperpolarize membranes by opening inwardly-rectifying K+ channels and attenuate the spontaneous release of L-glutamate from nerve terminals in the substantia gelatinosa, both of which are mediated by mu-opioid receptors, in a manner quantitatively similar to each other. The difference in antinociceptive effects between endomorphin-1 and endomorphin-2 could not be attributed to a distinction in their effects on excitatory transmission in substantia gelatinosa neurons, and may be explained by a difference in their enzymatic degradation.

Differential activation of G-proteins by mu-opioid receptor agonists

We investigated the ability of the activated mu-opioid receptor (MOR) to differentiate between myristoylated G(alphai1) and G(alphaoA) type G(alpha) proteins, and the maximal activity of a range of synthetic and endogenous agonists to activate each G(alpha) protein. Membranes from HEK293 cells stably expressing transfected MOR were chaotrope extracted to denature endogenous G-proteins and reconstituted with specific purified G-proteins. The G(alpha) subunits were generated in bacteria and were demonstrated to be recognised equivalently to bovine brain purified G(alpha) protein by CB(1) cannabinoid receptors. The ability of agonists to catalyse the MOR-dependent GDP/[(35)S]GTP(gamma)S exchange was then compared for G(alphai1) and G(alphaoA). Activation of MOR by DAMGO produced a high-affinity saturable interaction for G(alphaoA) (K(m)=20+/-1 nM) but a low-affinity interaction with G(alphai1) (K(m)=116+/-12 nM). DAMGO, met-enkephalin and leucine-enkephalin displayed maximal G(alpha) activation among the agonists evaluated. Endomorphins 1 and 2, methadone and beta-endorphin activated both G(alpha) to more than 75% of the maximal response, whereas fentanyl partially activated both G-proteins. Buprenorphine and morphine demonstrated a statistically significant difference between the maximal activities between G(alphai1) and G(alphaoA). Interestingly, DAMGO, morphine, endomorphins 1 and 2, displayed significant differences in the potencies for the activation of the two G(alpha). Differences in maximal activity and potency, for G(alphai1) versus G(alphaoA), are both indicative of agonist selective activation of G-proteins in response to MOR activation. These findings may provide a starting point for the design of drugs that demonstrate greater selectivity between these two G-proteins and therefore produce a more limited range of effects.

Comparison of the relative efficacy and potency of mu-opioid agonists to activate Galpha(i/o) proteins containing a pertussis toxin-insensitive mutation

Pertussis toxin (PTX)-insensitive mutants of Galpha(i/o) proteins expressed in C6mu cells were used to examine the hypothesis that there are agonist-specific conformational states of the mu-opioid receptor with coupling preferences to different Galpha(i/o) subtypes, as measured by the degree of stimulation of [(35)S]guanosine 5'-O-(3-thio)triphosphate (GTPgammaS) binding. Binding of [(35)S]GTPgammaS to endogenous Galpha(i/o) proteins stimulated by the full mu-opioid agonist [d-Ala(2),MePhe(4),Gly(5)-ol]enkephalin (DAMGO) was completely blocked by overnight treatment with 100 ng/ml PTX. Treatment for 4 h with lower concentrations led to a PTX-dependent reduction in the maximal effect of DAMGO but no alteration in the potency of DAMGO or morphine nor in the relative maximal effect (relative efficacy) of the partial agonists morphine and buprenorphine compared with the full agonist DAMGO. Using PTX-insensitive Galpha mutants in which the PTX-sensitive cysteine was replaced with isoleucine, the potency for a series of mu-opioid agonists was highest in cells expressing Galpha(i3) and Galpha(o) and lowest with Galpha(i1) and Galpha(i2), with no significant change in the order of potency, namely, etorphine >> endomorphin-1 = DAMGO = endomorphin-2 = fentanyl = morphine >> meperidine. The order of agonist relative efficacy, etorphine = DAMGO = endomorphin-1 = endomorphin-2 = fentanyl > or = morphine > or = meperidine > buprenorphine > or = nalbuphine, was also the same across all of the PTX-insensitive Galpha(i/o) subtypes. Highest relative efficacy to stimulate [(35)S]GTPgammaS binding was seen with Galpha(i3). Consequently, reported observations of agonist-directed trafficking at mu-opioid receptors most likely involve non-PTX-sensitive Galpha protein mechanisms.

Bring your own G protein

G protein-coupled receptor (GPCR)-Galpha fusion proteins were first characterized more than 10 years ago as a strategy for studying receptor-G protein signaling. A large number of studies have used this approach to characterize receptor coupling to members of the Gs, Gi, and Gq families of Galpha subunits, but this strategy has not been widely used to study Galpha12 and Galpha13. As described in the article by Zhang et al. in this issue of Molecular Pharmacology (p. 1433) characterization of the signaling properties of thromboxane A2 receptor (TPalpha) -Galpha12 and -Galpha13 fusion constructs demonstrates the applicability of this strategy to members of this unique family of Galpha subunits, and how this strategy can be used to resolve otherwise difficult problems of receptor pharmacology associated with these proteins. The general strategy of making receptor-Galpha fusion constructs has wide applicability to a number of research problems, but there are perhaps also "hidden messages" in how different receptor-Galpha subunit fusion pairs behave.

Functional complementation and the analysis of opioid receptor homodimerization

Complementation of function after coexpression of pairs of nonfunctional G protein-coupled receptors that contain distinct inactivating mutations supports the hypothesis that such receptors exist as dimers. Chimeras between members of the metabotropic glutamate receptor-like family have been particularly useful because the N-terminal ligand binding and heptahelical transmembrane elements can be considered distinct domains. To examine the utility of a related approach for opioid receptors, fusion proteins were generated in which a pertussis toxin-resistant (Cys351Ile) variant of the G protein Gi1alpha was linked to the C-terminal tails of the delta opioid peptide (DOP), kappa opioid peptide, and mu opioid peptide receptors. Each was functional as measured by agonist stimulation of guanosine 5'-([gamma-35S]thio)triphosphate ([35S]GTPgammaS) binding in Gialpha immunoprecipitates from membranes of pertussis toxin-treated HEK293 cells. Agonist function was eliminated either by fusion of the receptors to Gi1alphaGly202Ala,Cys351Ile or mutation of a pair of conserved Val residues in intracellular loop 2 of each receptor. Coexpression, but not simple mixing, of the two inactive fusion proteins reconstituted agonist-loading of [35S]GTPgammaS for each receptor. At equimolar amounts, reconstitution of DOP receptor function was more extensive than kappa or mu opioid receptor. Reconstitution of DOP function required two intact receptors and was not achieved by provision of extra Gi1alphaCys351Ile membrane anchored by linkage to DOP transmembrane domain 1. Inactive forms of all G protein alpha subunits can be produced by mutations equivalent to Gi1alphaGly202Ala. Because the amino acids modified in the opioid receptors are highly conserved in most rhodopsin-like receptors, this approach should be widely applicable to study the existence and molecular basis of receptor dimerization.

Effector antagonism by the regulators of G protein signalling (RGS) proteins causes desensitization of mu-opioid receptors in the CNS

RATIONALE

In cell culture systems, agonists can promote the phosphorylation and internalization of receptors coupled to G proteins (GPCR), leading to their desensitization. However, in the CNS opioid agonists promote a profound desensitization of their analgesic effects without diminishing the presence of their receptors in the neuronal membrane. Recent studies have indicated that CNS proteins of the RGS family, specific regulators of G protein signalling, may be involved in mu-opioid receptor desensitization in vivo.

OBJECTIVE

In this work we review the role played by RGS proteins in the intensity and duration of the effects of mu-opioid receptor agonists, and how they influence the delayed tolerance that develops in response to specific doses of opioids.

RESULTS

RGS proteins are GTPase-activating proteins (GAP) that accelerate the hydrolysis of GalphaGTP to terminate signalling at effectors. The GAP activity of RGS-R4 and RGS-Rz proteins restricts the amplitude of opioid analgesia, and the efficient deactivation of GalphazGTP subunits by RGS-Rz proteins prevents mu receptor desensitization. However, RGS-R7 proteins antagonize effectors by binding to and sequestering mu receptor-activated Galphai/o/z subunits. Thus, they reduce the pool of receptor-regulated G proteins and hence, the effects of agonists. The delayed tolerance observed following morphine administration correlates with the transfer of Galpha subunits from mu receptors to RGS-R7 proteins and the subsequent stabilization of this association.

CONCLUSION

In the CNS, the RGS proteins control the activity of mu opioid receptors through GAP-dependent (RGS-R4 and RGS-Rz) as well as by GAP-independent mechanisms (RGS-R7). As a result, they can both antagonize effectors and desensitize receptors under certain circumstances.

Mu opioid receptor coupling to Gi/o proteins increases during postnatal development in rat brain

Mu opioid receptors are densely expressed within rat striatum and are concentrated in anatomically discrete patches called striosomes. The density of striosomal mu receptors remains relatively constant during postnatal development, but little is known about their functional maturation. We examined the extent of G protein coupling by mu opioid receptors in rat brain during development, focusing on striosomes within the striatum because of receptor density. The mu receptors were quantified using [(3)H][d-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin (DAMGO) autoradiography. Adjacent sections were analyzed for DAMGO-stimulated guanosine 5'-O-(3-[(35)S]thio)triphosphate ([(35)S]GTPgammaS) binding to assess mu receptor activation of G(i/o) proteins. Striosomal mu receptor expression increased only slightly between postnatal day 5 and adult. In contrast, mu receptor-stimulated [(35)S]GTPgammaS binding increased from 0.13 to 2.6 fmol/mg tissue over the same period, a 20-fold difference. The ratio of specific DAMGO-stimulated [(35)S]GTPgammaS binding to [(3)H]DAMGO binding, representing the relative number of G proteins activated per receptor, increased 19-fold between postnatal day 5 and adult. Similar patterns were observed throughout the striatum and other brain regions such as the nucleus accumbens, although the extent of change varied from region to region. These data indicate that mu opioid receptors exhibit enhanced function in the adult rat brain compared with the neonate. These data also suggest that this increase in G protein coupling is developmentally regulated and that in the developing rat brain the density of mu opioid receptor expression may not necessarily correlate with receptor activation of G proteins.

RGS-Rz and RGS9-2 proteins control mu-opioid receptor desensitisation in CNS: the role of activated Galphaz subunits

Two consecutive i.c.v. administrations of analgesic doses of mu-opioid receptor agonists lead to a profound desensitisation of the latter receptors; a third dose produced less than 20% of the effect obtained with the first administration. Desensitisation was still effective 24h later. Impairing the activity of Galphaz but not Galphai2 subunits prevented tolerance developing after the administration of three consecutive doses of morphine. Further, the i.c.v. injection of Galphai2 subunits potentiated morphine analgesia and abolished acute tolerance, whereas i.c.v.-administered Galphaz subunits produced a rapid and robust loss of the response to morphine. The RGSZ1 and RGSZ2 proteins selectively deactivate GalphazGTP subunits, and their knockdown increased the effects produced by the first dose of morphine. However, impairing their activity also accelerated tachyphylaxis following successive doses of morphine, and facilitated the development of acute morphine tolerance. In contrast, inhibiting the RGS9-2 proteins, which bind to GalphaoGTP and GalphaiGTP but only weakly deactivates them, preserved the effects of consecutive morphine doses and abolished the generation of acute tolerance. Therefore, desensitisation of mu-opioid receptors can be achieved by reducing the responsiveness of post-receptor elements (via the possible action of activated Galphaz subunits) and/or by depleting the pool of receptor-regulated G proteins that agonists need to propagate their effects, e.g., through the activity of RGS9-2 proteins.

Biochemical demonstration of mu-opioid receptor association with Gsalpha: enhancement following morphine exposure

Biochemical data indicate mu-opioid receptor (MOR) coupling predominantly to the G(i) and G(o) family. Additionally, MOR coupling to G(s) is suggested by pharmacological assessments that have revealed excitatory MOR effects, which are resistant to pertussis toxin and sensitive to cholera toxin. However, biochemical evidence for such interactions remains elusive; G(salpha) has not been shown to be present in immunoprecipitate obtained using anti-MOR antibodies. In the current study, the presence of MOR in immunoprecipitate obtained with anti-G(salpha ) antibodies was investigated using Chinese hamster ovary cells stably transfected with MOR (MOR-CHO). MOR Western analyses of opioid naive MOR-CHO membranes immunoprecipitated using anti-G(salpha) antibodies reveal the presence of an approximately 75-80 kDa MOR species. Interestingly, acute and chronic morphine treatment markedly enhances the magnitude of MOR that co-immunoprecipitates with G(salpha), despite the concomitant down-regulation of membrane MOR protein. Enhanced co-precipitation of MOR with G(salpha) occurs without a concomitant increase in the immunoprecipitated G(salpha) protein indicating their increased association. In contrast, chronic morphine diminishes the co-immunoprecipitation of MOR with G(ialpha). Moreover, although only a single MOR species co-immunoprecipitated with G(salpha), MOR Western analysis of MOR-CHO membranes as well as immunoprecipitate obtained with either anti-MOR or anti-G(ialpha) antibodies reveals the presence of multiple molecular mass species of MOR. These data reveal the existence of a subset of MORs whose association with G(salpha) can be enhanced by morphine exposure. Notably, the regulation by chronic morphine of MOR association with G(salpha) and G(ialpha) is reciprocal. The relevance of MOR-Gs(alpha) coupling to opioid tolerance formation is discussed.

Mu-opioid receptor desensitization: is morphine different?

Opioid tolerance and dependence are important phenomena. The contribution of acute mu-opioid receptor regulatory mechanisms to the development of analgesic tolerance or physical dependence are unknown, and even the mechanisms underlying relatively rapid receptor desensitization in single cells are unresolved. To a large degree, the uncertainty surrounding the mechanisms and consequences of short-term regulation of tau-opioid receptors in single cells arises from the limitations in the experimental design in many of the studies that have investigated these events. Receptor overexpression and use of assays in which regulatory mechanisms are likely to blunt control determinations have led to measurements of opioid receptor activity that are likely to be insensitive to receptor uncoupling. Together with uncertainties concerning molecular details of tau-opioid receptor interactions with potential regulatory molecules such as G protein-coupled receptor kinases and arrestins, we are left with an incomplete picture crudely copied from the well-worked-out regulatory schema for beta(2)-adrenoceptors. As a consequence, suggestions that clinically relevant tau-opioid receptor agonists may have different propensities to produce tolerance and dependence that arise from their differential recruitment of regulatory mechanisms are premature, and have not yet been appropriately assessed, nor explained in the context of a thoroughly established regulatory scheme. In this commentary, we outline the experimental limitations that have given rise to conflicting ideas about how mu-opioid receptors are regulated, and identify the issues we feel still need to be addressed before we can understand why morphine promotes receptor trafficking differently to other opioids.

RGSZ1 and GAIP regulate mu- but not delta-opioid receptors in mouse CNS: role in tachyphylaxis and acute tolerance

In the CNS, the regulators of G-protein signaling (RGS) proteins belonging to the Rz subfamily, RGS19 (G(alpha) interacting protein (GAIP)) and RGS20 (Z1), control the activity of opioid agonists at mu but not at delta receptors. Rz proteins show high selectivity in deactivating G(alpha)z-GTP subunits. After reducing the expression of RGSZ1 with antisense oligodeoxynucleotides (ODN), the supraspinal antinociception produced by morphine, heroin, DAMGO ([D-Ala2, N-MePhe4,Gly-ol5]-enkephalin), and endomorphin-1 was notably increased. No change was observed in the effect of endomorphin-2. This agrees with the proposed existence of different mu receptors for the endomorphins. The activities of DPDPE ([D-Pen2,5]-enkephalin) and [D-Ala2] deltorphin II, agonists at delta receptors, were also unchanged. Knockdown of GAIP and of the GAIP interacting protein C-terminus (GIPC) led to changes in agonist effects at mu but not at delta receptors. The impairment of RGSZ1 extended the duration of morphine analgesia by at least 1 h beyond that observed in control animals. CTOP (Cys2, Tyr3, Orn5, Pen7-amide) antagonized morphine analgesia when given during the period in which the effect of morphine was enhanced by RGSZ1 knockdown. Thus, in naive mice, morphine tachyphylaxis originated in the presence of the opioid agonist and during the analgesia time course. The knockdown of RGSZ1 facilitated the development of tolerance to a single dose of morphine and accelerated tolerance to continuous delivery of the opioid. These results indicate that mu but not delta receptors are linked to Rz regulation. The mu receptor-mediated activation of Gz proteins is effective at recruiting the adaptive mechanisms leading to the development of opioid desensitization.

The [35S]GTPgammaS binding assay: approaches and applications in pharmacology

Receptors of the of seven transmembrane spanning, heterotrimeric G protein coupled family (GPCR) play crucial roles in regulating physiological functions and consequently are targets for the action of many classes of drugs. Activation of receptor by agonist leads to the dissociation of GDP from Galpha of the Galphabetagamma heterotrimer, followed by the binding of GTP to Galpha and subsequent modulation of downstream effectors. The G protein heterotrimer is reformed by GTPase activity of the Galpha subunit, forming Galpha-GDP and so allowing Galpha and Gbetagamma to recombine. The [35S]GTPgammaS assay measures the level of G protein activation following agonist occupation of a GPCR, by determining the binding of the non-hydrolyzable analog [35S]GTPgammaS to Galpha subunits. Thus, the assay measures a functional consequence of receptor occupancy at one of the earliest receptor-mediated events. The assay allows for traditional pharmacological parameters of potency, efficacy and antagonist affinity, with the advantage that agonist measures are not subjected to amplification or other modulation that may occur when analyzing parameters further downstream of the receptor. In general the assay is experimentally more feasible for receptors coupled to the abundant G(i/o) proteins. Nevertheless, [35S]GTPgammaS binding assays are used with GPCRs that couple to the G(s) and G(q) families of G proteins, especially in artificial expression systems, or using receptor-Galpha constructs or immunoprecipitation of [35S]GTPgammaS-labeled Galpha. The relative simplicity of the assay has made it very popular and its use is providing insights into contemporary pharmacological topics including the roles of accessory proteins in signaling, constitutive activity of receptors and agonist specific signaling.

Enhanced spontaneous activity of the mu opioid receptor by cysteine mutations: characterization of a tool for inverse agonist screening

Background

The concept of spontaneous- or constitutive-activity has become widely accepted and verified for numerous G protein-coupled receptors and this ligand-independent activity is also acknowledged to play a role in some pathologies. Constitutive activity has been reported for the mu opioid receptor. In some cases the increase in receptor basal activity was induced by chronic morphine administration suggesting that constitutive activity may contribute to the development of drug tolerance and dependence. Constitutively active mutants represent excellent tools for gathering information about the mechanisms of receptor activation and the possible physiological relevance of spontaneous receptor activity. The high basal level of activity of these mutants also allows for easier identification of inverse agonists, defined as ligands able to suppress spontaneous receptor activity, and leads to a better comprehension of their modulatory effects as well as possible in vivo use.

Results

Cysteines 348 and 353 of the human mu opioid receptor (hMOR) were mutated into alanines and Ala^348,353 hMOR was stably expressed in HEK 293 cells. [³⁵S] GTPγS binding experiments revealed that Ala^348,353 hMOR basal activity was significantly higher when compared to hMOR, suggesting that the mutant receptor is constitutively active. [³⁵S] GTPγS binding was decreased by cyprodime or CTOP indicating that both ligands have inverse agonist properties. All tested agonists exhibited binding affinities higher for Ala^348,353 hMOR than for hMOR, with the exception of endogenous opioid peptides. Antagonist affinity remained virtually unchanged except for CTOP and cyprodime that bound the double mutant with higher affinities. The agonists DAMGO and morphine showed enhanced potency for the Ala^348,353 hMOR receptor in [³⁵S] GTPγS experiments. Finally, pretreatment with the antagonists naloxone, cyprodime or CTOP significantly increased Ala^348,353 hMOR expression.

Conclusion

Taken together our data indicate that the double C348/353A mutation results in a constitutively active conformation of hMOR that is still activated by agonists. This is the first report of a stable CAM of hMOR with the potential to screen for inverse agonists.

Selective interactions between helix VIII of the human mu-opioid receptors and the C terminus of periplakin disrupt G protein activation

Analysis of interactions between the C-terminal tail of the MOP-1 and MOP-1A variants of the human mu-opioid receptor with proteins derived from a human brain cDNA library resulted in identification of the actin and intermediate filament-binding protein periplakin. Mapping of this interaction indicated that the predicted fourth intracellular loop/helix VIII of the receptor interacts with the C-terminal rod and linker region of periplakin. Periplakin is widely expressed in the central nervous system of both man and rat and demonstrated an overlapping but not identical distribution with mu-opioid (MOP) receptors. Co-expression of periplakin with MOP-1 or a MOP-1-eYFP fusion construct in HEK293 cells did not interfere with agonist-mediated internalization of the receptor. When co-expressed with a MOP-1-Gi1 alpha fusion protein periplakin significantly reduced the capacity of the agonist to stimulate binding of 35S-labeled guanosine 5'-3-O-(thio)triphosphate ([35S]GTP gamma S) to the receptor-associated G protein. By contrast, periplakin did not interfere with agonist-stimulation of [35S]GTP gamma S binding to either an alpha 2A-adrenoreceptor-Gi1 alpha fusion protein or a beta2-adrenoreceptor-Gs alpha fusion protein, indicating its selectivity of function. This represents the first example of an opioid receptor-interacting protein that functions to disrupt agonist-mediated G protein activation.

Endogenous opiates and behavior: 2002

This paper is the twenty-fifth consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over a quarter-century of research. It summarizes papers published during 2002 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).

G protein-coupled receptor overexpression with the baculovirus-insect cell system: a tool for structural and functional studies

G protein-coupled receptors, whose topology shows seven transmembrane domains, form the largest known family of receptors involved in higher organism signal transduction. These receptors are generally of low natural abundance and overexpression is usually a prerequisite to their structural or functional characterisation. The baculovirus-insect cell system constitutes a versatile tool for the maximal production of receptors. This heterologous expression system also provides interesting alternatives for receptor functional studies in a well-controlled cellular context.